git-gui: Make use of the Tk font system rather than faking it.

git-gui: Make use of the Tk font system rather than faking it.

The native Tk font system is actually quite powerful and has the nice
property that modifications to a named font are immediately reflected
throughout all widgets currently displayed.  This really saves us
from needing to write all of the reconfigure code as part of our font
display.

I also fixed the way we detect and apply the system font on the main
UI widgets as although it worked on a Windows 2000 system it does not
work at all on my Mac OS 10.4 system.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>