Make gitk save and restore the user set window position.

Make gitk save and restore the user set window position.

gitk was saving widget sizes and positions when the main window was
destroyed, which is after all child widgets are destroyed. The cure
is to trap the WM_DELETE_WINDOW event before the gui is torn down. Also,
the saved geometry was captured using "winfo geometry .", rather than
"wm geometry ." Under Linux, these two return different answers and the
latter one is correct.

[jc: credit goes to Brett Schwarz for suggesting the use of "wm protocol";
 I also squashed the follow-up patch to remove extraneous -0
 from expressions.]

Signed-off-by: Mark Levedahl <mdl123@verizon.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>