gitk: Keep track of font attributes ourselves instead of using font actual

gitk: Keep track of font attributes ourselves instead of using font actual

Unfortunately there seems to be a bug in Tk8.5 where font actual -size
sometimes gives the wrong answer (e.g. 12 for Bitstream Vera Sans 9),
even though the font is actually displayed at the right size.  This
works around it by parsing and storing the family, size, weight and
slant of the mainfont, textfont and uifont explicitly.

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