Make Full Screen action use consistent text and icon

This is a checkable action, and right now when unchecked, it shows "Full
Screen Mode" but when checked, it shows "Exit Full Screen Mode". The
icon changes too.

Changing the text when checked does not make sense and violates the HIG
(https://develop.kde.org/hig/components/editing/togglebutton/#behavior).

BUG: 240854
FIXED-IN: 5.101
11 jobs for !160 with make-fullscreen-action-use-consistent-text-and-icon in 9 minutes and 44 seconds (queued for 5 seconds)
merge request