git-gui: set suitable extended window manager hints.

This patch uses recent Tk attributes support to specify the intended use of new
toplevels by setting the correct EWMH hint. This helps modern window managers
to apply sensible decoration for the tooltip and dialogs.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

lib/themed.tcl

@@ -123,6 +123,7 @@ proc paddedlabel {w args} {
 # place a themed frame over the surface.
 proc Dialog {w args} {
 	eval [linsert $args 0 toplevel $w -class Dialog]
+	catch {wm attributes $w -type dialog}	
 	pave_toplevel $w
 	return $w
 }