Deprecate _drawing.scss and _colors.scss (GNOME shell/Cinnamon)

[hrdwrrsk]

Since widgets like buttons are now using SVG asset instead of CSS (due to limitations in St toolkit), _drawing.scss file used in GNOME shell/Cinnamon are almost obsolete and no longer used, but it's still used in some areas currently. Also _colors.scss will be merged into Gtk version instead using separate _colors.scss

some tasks:

• [ ] Add entry/textfield assets in GNOME shell theming
• [ ] Remove widget drawing import from _drawing.scss in _common.scss
• [x] check and compare _colors.scss before it uses _colors.scss from Gtk