[editor] "Can’t edit glyph" dialog should be refined

[justvanrossum]

If the editor is not "at a source", and the user tries to move a point or component, we get a dialog that says:

Can’t edit glyph “<name-of-glyph>”
Location is not at a source.
                  (( Okay ))

What it should do instead is:

Can’t edit glyph “<name-of-glyph>”
Would you like to create a new source or navigate to the nearest source?
  ( Cancel )  ( Create new source )  (( Go to nearest source ))

[fatih-erikli]

I receive an error in console when I try to edit a glyph that it's source deleted.

varGlyph.sources[glyphController.sourceIndex] is undefined