Add *.xpm and *.c bitmap choices to FontForge

Seemed *.xpm was partially finished, therefore added it.
There is also a function for building internal FontForge images and now
that it's working okay, added that also to the bitmap export menu too.
Merge pull request #546 from JoesCat/Add_2_Bitmaps

Add *.xpm and *.c bitmap choices to FontForge