Titlebar Color Sync and Rounded Corners

(Video on Mastodon)

Inspired by a Reddit post some month ago, I wrote two patches:

1. titlebar-colors.patch for syncing the colors of the titlebar with the rest of the frame. This uses GtkHeaderbar to allow Emacs to control the colors internally. I haven't gotten the foreground color to sync yet. The changes are done to GTK parts of Emacs and should work for both X and Wayland (pgtk) builds.
2. rounded-corners.patch to make the shape of frames more consistent with other applications. The changes are made to PGTK parts of Emacs, but should be easily applicable to other places. Right now, it forces a 12 px radius. Ideally, it should just respect the shape of the underlying GtkWindow. This would make it more versatile and automatically change to a square shape in fullscreen (which is currently unimplemented).

The Reddit post mentioned got a lot of attention, but the poster did not provide any code. So I thought some people would find at least the first of these patches useful!

They both have their flaws (poor contrast in most dark themes and forcing 12 px radius, also seems to crash Corfu and possibly more), any help to improve them is much appreciated.