Add Drag and Drop support

[grymmjack]

Electron supports drag and drop. It would be nice if we could drag and drop files onto moebius.

*.ans, *.xb, *.asc, etc. text mode documents could open in a new window.
*.f?? fonts could load the font.
*.gpl (gimp palette) could load color palettes.

See: https://www.electronjs.org/docs/latest/tutorial/native-file-drag-drop