Support New UI

[Foldex]

Figured I might as well open it.

TODO:

• Top/Bottom Bars
• Overlay
• Notifications
• Settings
• Game Properties
• Add Non-Steam Game
• Product Activation
• Recent Players
• Game Servers
• Screenshots
• About Steam
• System Info

[Foldex]

Install Dialog done:

Overlay List Mode done:

Steam Settings is partially cleaned up, though it's not quite Adwaita.
Due to how many sub layouts are used within this dialog, most of which using generic classes, getting things to look like Adwaita Preference Rows without breaking another view is likely going to be a pain:

For those who don't like the Adwaita styled Top Bar, added a web extra that reverts to the Original Steam Bar:

[Blisterexe]

apologies if you already know this, but the steam close button (middle) does not look like it should, at least compared to native (themed using gradience) gtk4 applications

[Foldex]

the steam close button (middle) does not look like it should

Latest Beta Client made some changes to the Window Controls.
Should be fixed by ace865d

[Foldex]

@tkashkin
New UI is out of Beta, so marking this as ready to review.