gitk: Use named fonts instead of the font specification

gitk: Use named fonts instead of the font specification

This replaces the use of $mainfont, $textfont and $uifont with named
fonts called mainfont, textfont and uifont.  We also have variants
called mainfontbold and textfontbold.  This makes it much easier to
make sure font size changes are reflected everywhere they should be,
since configuring a named font automatically changes all the widgets
that are using that font.

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