Alert if font is being used on delete of font (Bug 1307240)

Now gives the count of how many skins the font is attached to if it is
in fact attached to a skin.

And on deletion remove the font from the skin

Change-Id: I8784f5ff83df4adaffe4c6acd50dd44e1231124b
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>