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>
diff --git a/lib/class.tcl b/lib/class.tcl
index c27b714..f08506f 100644
--- a/lib/class.tcl
+++ b/lib/class.tcl
@@ -138,6 +138,7 @@
 	upvar $t top $w pfx this this
 	global use_ttk
 	uplevel [linsert $args 0 make_toplevel $t $w]
+	catch {wm attributes $top -type dialog}
 	pave_toplevel $pfx
 }