Views: supply own headerbars

[danirabbit]

There's still a lot we can do to make better use of space in views like AppInfoView, but this gives us a canvas to pursue those things at least

Todo:

• [x] Add headerbars to all the views
• [x] Move updates settings to updates view
• [x] Category title on category views
• [x] Back buttons

[zeebok]

Not sure exactly when it changed but the readme needs to be updated to need libportal-gtk4-dev