Link mode buttons together
Make shoot action button wider
Use symbolic icon for leave fullscreen button
Remove mode and shoot actions from the app menu

The current GNOME HIG suggests that actions which are already accessible
from the application window should not be duplicated in the application
menu:

https://wiki.gnome.org/Design/HIG/ApplicationMenus
Convert some GtkGrid widgets to GtkBox
Add a border around the button area box
Remove unused countdown_action
Change titlebar to use GtkHeaderBar

https://wiki.gnome.org/GnomeGoals/HeaderBars

https://bugzilla.gnome.org/show_bug.cgi?id=711714