save and restore system settings when moving between menu and script

encoder, key and screen settings should be saved and restored between switches.

specifically, this will

simplify script redraw()s since scripts won't need to (re)assert their screen settings
allow scripts to customize encoder and key behavior

forward pointers:

menu.init() asserts encoder settings here:

which is called after script init here:

see also: #345 (for the plumbing to manage screen state).

/cc @tehn @artfwo

monome / norns