Port to GTK4

The following modules have been ported:

• Application
• Window
• TextView
  • Markup handler
• Headerbars
  • Open Popover
  • Preview Layout Switcher
  • Theme switcher
• Search and replace
• Stats handler
• Export Dialog
• Preview panels
• About Dialog
• Preferences Dialog

Undo/redo handler and DnD handler were not longer necessary thanks to GTK4 and have been removed.

Stats handler now support selection stats Hemingway mode has gained visual feedback Styles have been improved, and have been ported to CSS Some animations have been ported to AdwAnimation

Spellcheck is blocked on GNOME/gtk#3814

Closes #423 (closed) Closes #356 (closed)