git-gui: Cleanup common font handling for font_ui
An earlier change tossed these optionMenu font configurations
all over the code, when really we can just rename the proc to
a hidden internal name and provide our own wrapper to install
the font configuration we really want.
We also don't need to set these option database entries in all
of the procedures that open dialogs; instead we should just set
one time, them after we have the font configuration ready for use.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>