* 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
Right now users are confused when various VTE actions don't work because
there's no indication that the terminal is in the non-clean state.
Visualise "modified" terminal in the same way as modified document - by
a red label in the tab so it's clearer when terminal isn't clean.
Avoid quick red flashes when pressing enter by delaying the color change
a bit.
Also remove extra padding around the button's image to reduce its size
and try and avoid it expanding the tab's height.
This at least fixes the editor tabs height on Adwaita theme, where they
were 1px taller than normal.
