AccelGroup/AccelMap is phased out in GTK3.

Use actions instead because they can also have shortcuts. This makes
the code much easier to read because we dont have to deal with keyval and key modifiers.

Handle all actions that are not control specific inside the MessageWindow and
delegate all control specific actions to the controls.

Use a Stack page to display the change nickname entry instead of a popup dialog

Groupchats are bookmarked on join

- Add account selector
- Add toggle button for global search
- Disco MUCs found via global search
- Light refactoring

If there is only one account active we use the GroupchatJoin, else the jid
is passed to StartChat

- Remove usage of instance dict

Use only participant on user facing strings

Fixes #9755

https://github.com/flatpak/flatpak/issues/2031

Fixes #8955

- Make jid property a JID
- Add occupant_jid property
- nbxmpp: Use new muc_jid attribute

- When we join a MUC add a bookmark with autojoin enabled
- When we leave a MUC modify the bookmark and disable autojoin

Fixes #9754