Support all savefile types for DnD

[JohannesLorenz]

When dropping an external file on zest, the file is loaded. The code in script.rb accepts all file types, but it currently only makes sense for xmz-files.

The code should

• only accept drops if the savefile is over a suiting widget (e.g. LFO presets can not be dropped on envelopes)
• already inform X11 whether the pointer is over a widget where it would be possible to drop. See onDndTargetAcceptDrop in the pugl documentation.