Ui
UI updates, specially to the headerbar:
- Fix margin on the recent files popover
- Add the full path of the current file to the headerbar as subtitle and tooltip
- Rework the autohiding mechanism; now the headerbar fades away when typing, only to reappear when the cursor moves to the top portion of the window
- Now the content of the texview goes visually bellow the headerbar
- Rework the layout of the headerbar: now we have dedicated dropdowns for "save as/export" actions, and a new preview layout switcher
- Add theme selector to the appmenu