GtkSidebar: Added GtkSidebar implementation when GTK does not support it.
Preferences: Reworked the prefs window.

- GTK3 port.
- Use GmEntry where possible.
Roster: Fixed signal emission.
Call History: Connect signal to the main window.

This allows regenerating the main window menu when the selection &
actions change.
GActorMenu: Put different stores in different menu sections.
Call History: Cleanup.