Rework Preferences Window

This is an attempt at reworking the preferences window.

What is done so far:

• Restructured code in preferences.py to better reflect window layout
• Removed useless ids in preferences_window.ui
• Use Stack with StackSidebar instead of Notebook for window structure
• Integrate 'Chat State Notifications' in 'General' page
• Various layout improvements
• Fix #8722 (closed)
• Fix #7562 (closed)
• Fix #6820 (closed)
• Fix #4364 (closed)
• Fix #9397 (closed)
• Add setting for print_status_in_muc
• Fix duplicate mnemonics for each page (and add missing widget connections for combo boxes)
• Rework helper text strings and some labels
• Fix applications_frame missing
• Make use of new GtkBuilder API
• Restructure preferences by adding 'Chat' and 'Privacy' pages
• Add print_status_in_chat option