Add classvar ESCAPE_TO_MINIMIZE

[willmcgugan]

Fixes #4949

[darrenburns]

As mentioned, maybe this should be union of true/false/none. The value at App takes precedence. If the value is set (not None) at the Screen level, the the value from the Screen overrides.

[darrenburns]

Looks good. Just as a general question - is there something about our binding system that means we're preferring to handle this inside on_key rather than as a binding (or priority binding)? If we're opting to do this, it might suggest we're missing some functionality in the binding system?

[willmcgugan]

It could be done with a binding. But it would place a burden on the dev not to break it, and to do the right thing if they want to bind escape for something else. Maybe we could add something to the binding system to make that a non-issue in the future, but I don't know what that would look like.