WIP: Pixbuf cache

This creates a Pixbuf Hashmap<(filepath, width), Pixbuf> that keeps a refference to created Pixbufs to avoid constant re-creation. Note that this might lead to "leaked"(but, not quite) memory since there's no way as of now to remove a pixbuf from the cache. Thus the pixbuf will stay in memory even if the users leave the room.

I would like for the key to be some kind of ID, like an i32 from where we could figure out the file-path instead of String.

Note: The cached map should never be accessed from a different thread than the one that it was created. Send-Cell is a workaround cause GObjects do not implement Send trait.