gitk: replace ${NS} with ttk

gitk uses ${NS} to select between the original Tk widgets and the newer
themed widgets in ttk.  As gitk uses only themed widgets from ttk::,
this indirection now serves no purpose, so let's switch to explicit use
of ttk:: via global search/replace. More simplification, including
removal of the NS variable, is kept for a later patch to keep this one
smaller.

Signed-off-by: Mark Levedahl <[email protected]>

Author: mark987