Gtk4 Prep

[danirabbit]

Backports from #200 Can be rebase merged

Application

• Rename CSS file. It will be automatically loaded with Granite.init
• Can't do geometry hints
• Can't set window position
• bind maximize here

MainWindow:

• replace gtk.current_event_time with Gdk.CURRENT_TIME
• Use child property
• Code style
• explicit margins

CameraView:

• Rename main_widget to stack so it's clear what this widget is
• Organize
• Dialog uses present/response not run

TimerButton:

• Construct with a box since we can't do always_show_image in gtk4
• Organize
• Fix icon being misaligned