Deleting a font source should show warning about possible consequences

[justvanrossum]

Deleting a font source can cause:

• invalidated kerning (kerning refers to font source identifiers)
• invalidated glyphs (glyph sources .locationBase may refer to font source identifiers)

I think we should show a dialog when the users deletes a font source.

Proposed message:

• Headline: Are you sure you want to delete the selected font source?
• Message: Deleting a font source may result in kerning being lost or glyphs to become invalid.

Button titles:

• Cancel
• Delete