gitk: restore ui colors after cancelling config dialog

gitk: restore ui colors after cancelling config dialog

gitk provides a dialog to configure many ui colors. Any color element
changed in the dialog takes immediate effect before closing the dialog.
While cancelling the dialog after changing one or more colors avoids
saving the modified colors, the user must restart gitk to restore the
prior color set. This unfortunate behavior results because gitk does not
have a single routine to update all of the ui colors. The prior commit
eliminated the key impediment to having such a routine. So, let's create
a routine to update all configured colors at once, use this when
modifying colors, and also invoke this after restoring the prior set if
the dialog is cancelled.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>