gitk: Use themed tk widgets

With Tk 8.5+, this uses the themed widgets to improve the appearance
on Windows and MacOSX.  On X11 less difference is apparent, but users
can select alternate themes by setting *TkTheme in the resource
database (eg: *TkTheme: clam).

With Tk 8.6 there is a built-in font selection dialog.  This will make
use of that when available, as on Windows and MacOSX it calls the
native font selection dialog.

[paulus@samba.org - folded in subsequent patch to restore saved
pane sizes for ttk widgets, and trimmed trailing whitespace.]

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