gitk: Use a tabbed dialog to edit preferences

gitk: Use a tabbed dialog to edit preferences

This commit converts the user preferences dialog into a tabbed property
sheet grouping general properties, colours and font selections onto
separate pages. The previous implementation was exceeding the screen
height on some systems and this avoids such problems and permits extension
using new pages in the future.

If themed Tk is unavailable or undesired a reasonable facsimile of the
tabbed notebook widget is used instead.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>