Styles: add Gtk.Video

[danirabbit]

Fixes #804

Kooha-2025-04-10-15-31-32.mp4

• Add Gtk.Video to Demo
• Adjust provider priority. Fixes a bug where .osd styles were way too OP and couldn't be overridden
• Move .circular because this isn't just a thing that applies to buttons

Button:

• Make sure we set color in button.osd so that it doesn't inherit a non-contrasty color
• reset margins in button.osd instead of using expensive :not selector in .osd
• Add the same fun bounce transition we use in window controls and popovers to OSD buttons, but not when they're linked

Scale:

• add a transition for opacity to sliders
• Set rounded radius from fill origin, but make sure it's still square on the end

Slider:

• Don't use 50% border-radius in case our slider is non-square

Video:

• Add Styles for Gtk.Video and Gtk.MediaControls based on the custom mediacontrols we did for Videos
• Make sure video widgets on a square still get a black letter/pillarbox
• Use tabular numbers so the scale position doesn't jump around
• In this context, Fill must mean we're downloading a video so add a fun pulse animation here. I'm not sure if Gtk.Video actually uses this, I just messed around in inspector. I can remove this if it's excessive 😅

[wpkelso]

🚀