gitk: set config dialog color swatches in one place

gitk: set config dialog color swatches in one place

gitk's color selection dialog uses a number of "label" widgets to show
the current value of each selectable color. This uses the -background
color property of label widgets, and this property is overwritten when
the full ui color set is refreshed. The swatch colors are set
individually using code passed into the chooser dialog, so there is no
common routine to set all after updating the global ui colors.

Let's replace this with a single routine that does set all swatches,
removing a key impediment to restoring the ui colors if the dialog is
cancelled.

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