1. Check if the message has the muc#user namespace
2. Check if we have this jid already in the DB
3. Last resort disco the jid

The server sends us carbons if one of our other resources has
a conversation in a muc we are not joined.
Instead we log the messages silently.

Fallback must be happening in init_emoticon() instead of PreferencesWindow

- Use ComboBoxText, its much simpler
- Add a dedicated method that returns all available themes
- If the configured Theme is not available fallback to font-emoticons

This gives the server the chance to react accordingly without applying
much logic.
Also it makes it easier for us to recognize MUC PMs in MAM querys

Fixes #8812

We limit the search to non-groupchat messages because a stanza id
is only unique within an archive

On some Desktops eg. Mate, the MessageTextview was to big, because
the default height seems to be 82.

What influences the default height (compare Windows where it is 46),
is unclear.

Fixes #8901

Fixes #8907

- Makes it better understandable
- Fixes a bug where an avatar was drawn in a group row
- Fixes not all cells getting the same background color while using a theme

Fixes #8901

Fixes #8902

Fixes #8894

Avatars are now Surfaces, Gtk.Window has no method yet to add Surfaces
as icons.

Transforming them back to pixbufs is not viable because pixbufs cant hold
the scale information, and we have no way to set it.

This was only used with single chat windows, which is a mode that came
out of style and is probably not used often anymore.

Fixes #8895

https://valadoc.org/gtk+-3.0/Gtk.Button.set_use_underline.html

Delete some already ported methods

- use `with` statement and open() instead of os.open()
- execute run() on dialog, otherwise sys.exit() is called immediatly

getTimestamp() generates always a timestring with a resolution of seconds and
this means time gets always rounded down.
Because we use a resolution of .00000 precision for outgoing messages this
can lead to wrongly sorted messages.

Flatpak: bundle plugin installer

See merge request gajim/gajim!207