feat(helium): switch to the GTK theme with KWin title buttons

Helium now takes its colors from Breeze GTK instead of a generated
theme extension, which reset the GTK theme on every launch.

- Import window_decorations.css from gtk.css so Helium, which doesn't
  load kde-gtk-config's GTK module, draws the KWin title buttons
  instead of Breeze GTK's round ones.
- Add chromium.css to give the compact layout's address bar and
  button segment the title bar color, so the tab strip is one color.
- Disable WaylandWpColorManagerV1 so KWin doesn't lift Helium's dark
  grays (40 -> 44) and the strip matches KDE title bars.
- Drop the Helium theme from build.py and the helium package.

kde/helium/.config/helium-browser-flags.conf

@@ -2,5 +2,7 @@
 --ozone-platform-hint=auto
 --enable-features=AcceleratedVideoDecodeLinuxGL,AcceleratedVideoDecodeLinuxZeroCopyGL,AcceleratedVideoEncoder
 
-# Darwin Breeze theme, generated by kde/theme/build.py
---load-extension=~/.local/share/helium-themes/darwinbreeze
+# Don't tag surfaces as sRGB. KWin shows tagged sRGB through a gamma 2.2
+# curve, which lifts dark grays (40 -> 44), so the tab strip would come out
+# lighter than KDE title bars.
+--disable-features=WaylandWpColorManagerV1