Add help text for key combos for on import file browser screen

[maks]

The change to using physical keys vs on screen buttons have disorientated existing picoTracker and LGPT users so I think it would be useful to add a "help text" line on the bottom of the import screen showing that the play key is for previewing samples and LT+Play is to import a sample.

I have in mind something like:

PLAY->Preview  PLAY+LT->Import