gitk: Add a font chooser

gitk: Add a font chooser

This adds buttons to the edit preferences window to allow the user to
choose the main font, the text font (used for the diff display window)
and the UI font.  Pressing those buttons pops up a font chooser window
that lets the user pick the font family, size, weight (bold/normal)
and slant (roman/italic).

Signed-off-by: Paul Mackerras <paulus@samba.org>