gitk: Use consistent font for all text input fields

gitk: Use consistent font for all text input fields

Instead of setting the font for specific widgets, set the font for the
widget type. If themed widgets are not available, this is via the X
resources. If themed widgets are available, the theme font is used.

The exception is the SHA1 ID which is forced to use the fixed-width
font, even where themed widgets are used.

Signed-off-by: Mark Hills <mark@pogo.org.uk>
Signed-off-by: Paul Mackerras <paulus@samba.org>