focus/defocus user callbacks

[tehn]

some scripts might want to know when you enter/leave the menu

proposed user callback names:

• focus
• defocus

question: should these be called when a user script calls the fileselect or textentry?

alternatively:

• focus(state) could just have the to-be-entered state as an arg ("menu", "script", "fileselect", etc)

[tyleretters]

i like the alternative focus(state) with perhaps the addition of boolean check? is_focus(state)

[tyleretters]

(and fileselect and textentry could be refactored to call focus(thing))