Draft: Image pasting from clipboard, attaching MIME type information to allow previewable images and more

The goal of this MR is twofold:

• Allow pasting images #824
• Attach MIME type to outgoing messages #880 (needs to be changed in libcmatrix)

However, a larger amount of various fixes and improvements has also made it in. I can split those off into a different MR to ease review, if desired.

After digging a bit in the relevant parts of the codebase (ChattyFile, ChattyAttachment, ChattyAttachmentBar, ...) I have formulated a plan of attack.

ChattyFile

problems:

• currently only handles GFiles
• in most cases (except chatty_filew_new_full(): needs to do a bunch of {a,}sync stuff before it can be considered "ready for use"

solution: needs to become a bit smarter:

• needs to work with the clipboard
• use properties for mime-type, height, width etc, so that e.g. ChattyAttachment does not need to deal with this
• implement GInitable (and GAsyncInitable) instead of emulating it. (Currently we retrieve e.g. GFileInfo and bail on error, see e.g. chatty_file_new_for_path ()) After initialization is done, all relevant properties should have sensible values, or: even better, checked against provided expected values (in the case of chatt_file_new_full())
• Regarding constructors: I've opted /currently/ to have chatty_file_new_full() not do any initialization/validation. The underlying assumption being perhaps, that if you use _new_full() you've checked that e.g. it actually refers to a valid file - I may yet reconsider and have it use g_initable_new() (where all the checks happen) instead of g_object_new() for both consistencies sake as well as the added validations performed, which would also allow us to:
• Have more checks in place: expected size, mime-type, ... (another reasaon for g_initable_new()
• might need to add chatty_file_save_to_file(const char *path) for e.g. clipboard contents as otherwise the ChattyHistory will not be able to load older files (probably only relevant for MMS and perhaps XMPP, TODO need to check the libcmatrix side as well)
• tag all tasks
• Needs tests added!
• Do we maybe want to keep the GdkPixbuf around that we create for images? memory consumption? allow unloading?

potential constructors

sort of

ChattyFile       *chatty_file_new_finish         (GAsyncResult        *res,
                                                  GError             **error);
void              chatty_file_new_for_cm_event   (gpointer             cm_event,
                                                  GCancellable        *cancellable,
                                                  GAsyncReadyCallback  callback,
                                                  gpointer             user_data);
void              chatty_file_new_for_clipboard  (GCancellable        *cancellable,
                                                  GAsyncReadyCallback  callback,
                                                  gpointer             user_data);
void              chatty_file_new_for_path       (const char          *path,
                                                  GCancellable        *cancellable,
                                                  GAsyncReadyCallback  callback,
                                                  gpointer             user_data);
ChattyFile       *chatty_file_new_for_path_sync  (const char          *path);
ChattyFile       *chatty_file_new_full           (const char          *file_name,
                                                  const char          *url,
                                                  const char          *path,
                                                  const char          *mime_type,
                                                  gpointer             data, // maybe GBytes?, however we had size already, so ...
                                                  gsize                size, // I think both would still be fine, we can use that to error out
                                                  int                  width,
                                                  int                  height,
                                                  int                  duration);

ChattyAttachment

• Switch from GFile to ChattyFile
• Move thumbnail creation/retrievable logic to ChattyFile (probably sensible) Looking at it some more. Or perhaps chatty-media.c which has a similar ish

ChattyAttachmentBar

• Allow multiple attachments for matrix: We can just send multiple messages with one attachment each (follow-up material)

ChattyMessageBar

• when pasting anything other than text/plain create a new ChattyFile and add it to ChattyAttachmentBar

ChattyMaChat

• needs libcmatrix with new API
• needs to adapt in `chatty_ma_chat_message_send_async()