implement room history and reimplement sidebar

TO-DO list:

• Create GObjects from matrix events
• Create ListModel used in GtkListView
• Support selection of messages: Let's do it later: https://gitlab.gnome.org/GNOME/fractal/-/issues/742
• Add support for all message events (https://gitlab.gnome.org/GNOME/fractal/-/issues/739)
• Add support for all state events (https://gitlab.gnome.org/GNOME/fractal/-/issues/739)
• Show redacted events
• Support deletion of messages
• Support modification of messages
• Per room history
• Add context menu to allow operation on messages: delete, edit, reply forward etc. Mockup Let's do it later: https://gitlab.gnome.org/GNOME/fractal/-/issues/745
• Insert new message divider into the ListModel Let's do it later: https://gitlab.gnome.org/GNOME/fractal/-/issues/743
• Add empirical data to room history: eg. read receipt Let's do it later: https://gitlab.gnome.org/GNOME/fractal/-/issues/743
• Implement widget for new message divider
• Insert time divider into the ListModel
• Implement widget for and time divider
• Add support for message grouping (like we have now in fractal)
• Use matrix user avatars for events Needs a better AdwAvatar, Let's do it later in a separate MR
• Use matrix user display name for events. Fixes: https://gitlab.gnome.org/GNOME/fractal/-/issues/740
• Load previous messages. Some what blocked by https://github.com/matrix-org/matrix-rust-sdk/issues/178
• Allow changing the room from the sidebar
• Cleanup commit history

This MR is getting really big, therefore I split out some of the tasks.