Moves the headerbar into the separate deck pages (PlayerPage, WelcomePage, LibraryPage, EpisodesPage) and makes it flat. This allows for smoother transitions between pages and keeps Videos more inline with the general OS look
Use Hdy.ApplicationWindow for rounded corners
Do some gtk4 prep while we're here
The rounded corners are a bit buggy on the player, #335 might fix that though
Hdy.ApplicationWindowfor rounded cornersFixes #283 Fixes #232
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