GPX Notes test version

This allows choosing between OSM notes and GPX when creating a note. OSM will behave like usual; GPX will not remove the involved quest (if any) and add the note to the file notes.gpx in /Android/data/de.westnordost.streetcomplete/files
The file is immediately written when creating a note, without any further checks. So there is a chance of crashing if the file can't be written (never happened to me).

Images are completely ignored, better don't try using them when creating a GPX note (I didn't check, and they might just be stored inside some SC folder for all eternity).