This ensures that all users of the Mono flavour of Godot 3.0 are aware
of its current shortcomings (no export, crashes and usability issues).
The dialog is shown each time the editor is started, until the checkbox
is disabled (i.e. until users will have actually read it).
Fixes#15956.
Using `misc/scripts/fix_headers.py` on all Godot files.
Some missing header guards were added, and the header inclusion order
was fixed in the Bullet module.
Rémi Verschelde
Ignacio Etcheverry