New unified variables and variables declaration in GDevelop 5.4

This is how i would solve current problem

It would not penalize ones that do not want to declare their variables
While it would help ones that are into declaring their variables

If we want convince users they should declare their vars i say this is right path
Where we do all of this in same exact window we start to use new var
No one gets harmed everyone benefits from it
Nothing is taken away something is given to everyone