git-gui: Font chooser to handle a large number of font families

git-gui: Font chooser to handle a large number of font families

Simon Sasburg noticed that on X11 if there are more fonts than can
fit in the height of the screen Tk's native tk_optionMenu does not
offer scroll arrows to the user and it is not possible to review
all choices or to select those that are off-screen.  On Mac OS X
the tk_optionMenu works properly but is awkward to navigate if the
list is long.

This is a rewrite of our font selection by providing a new modal
dialog that the user can launch from the git-gui Options panel.
The dialog offers the user a scrolling list of fonts in a pane.
An example text shows the user what the font looks like at the size
they have selected.  But I have to admit the example pane is less
than ideal.  For example in the case of our diff font we really
should show the user an example diff complete with our native diff
syntax coloring.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Acked-by: Simon Sasburg <simon.sasburg@gmail.com>