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.
