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/theme/.config/gtk-3.0/chromium.css

@@ -0,0 +1,12 @@
+/* Helium (Chromium) with its GTK theme. Chromium adds a "chromium" class to
+   every node of the GTK style contexts it reads colors from, so these rules
+   reach Helium and no other GTK app.
+
+   In the compact layout, the address bar takes the text field color and the
+   segment with the extension and window buttons takes the window color,
+   which Chromium resolves to the darker view color. Both get the title bar
+   color, so the strip reads as one bar like a KDE title bar. */
+window.background.chromium,
+textview.view.chromium {
+  background-color: @theme_bg_color_breeze;
+}