Theme improvements (#1382)

Theme improvements (#1382)

* Improve GTK+3 theme loading

- Merge geany-3.0.css into geany.css
- Load geany-3.20.css to override if runtime version dictates
- Load geany.css from app->configdir if it exists.

* Add initial documentation for GTK+ CSS theming

* Add geany.css file to the `Tools->Configuration Files` menu

Also mark such config files as changed if they don't already exist to
force the user to save or discard any newly created config files.

* Allow customizing message window using GTK+ themes

* Improve GTK+3 CSS docs a bit

State which classes are meant to be overridden by themes.

* Go back to conditionally loading geany-3.0.css

It can't be merged into geany.css since GTK+ CSS parser doesn't ignore
invalid properties when prefixed with vendor extension (ie. -Gtk).

* Don't mark unedited config files as changed

They probably should be marked as changed since they don't exist on
disk yet, but for the time being leave it how it was.

* Fix misuse of CSS ID vs class in manual

* Minor colour value tweaks

Based on feedback from @b4n