Prepare cinnamon theme variants; closes #3754

- set cinnamon component option to true
- add colorable assets file
- Update the meson build script to create variants
- Follow the way gnome-shell variants are created
- Not using directories cinnamon, cinnamon-dark, dark and
  default. Can be cleaned up later.

cinnamon-shell/meson.build

@@ -1,5 +1,7 @@
+cinnamonshell_default_theme_dir = join_paths(get_option('datadir'), 'themes')
+cinnamonshell_alt_themes_dir = cinnamonshell_default_theme_dir
 flavours = []
-foreach flavour: ['default', 'dark', 'cinnamon', 'cinnamon-dark']
+foreach flavour: ['default', 'dark']
   if not get_option(flavour)
     message('skip flavour ' + flavour)
     continue

cinnamon-shell/src/cinnamon-shell.scss.in

@@ -0,0 +1,11 @@
+$variant: if(@DarkVariant@, 'dark', 'light');
+$yaru_variant: '@YaruVariant@';
+
+@import 'sass-utils';
+
+@debug 'Generating Cinnamon Shell ' + $yaru_variant + ' theme (' + $variant +' variant'+')';
+
+@import "@VariantDir@/_@Colors@";
+@import "@VariantDir@/_drawing";
+@import "@VariantDir@/_common";
+@import "@VariantDir@/_tweaks";