turn on auto-mnemonics (gtk 2.20 feature)

build/etc/gtk-2.0/gtkrc

@@ -19,6 +19,9 @@ gtk-alternative-sort-arrows = 1
 # Windows users don't expect the PC Speaker beeping at them when they backspace in an empty textview and stuff like that
 gtk-error-bell = 0
 
+# hide mnemonic underlines until the Alt key is pressed
+gtk-auto-mnemonics = 1
+
 style "msw-default"
 {
   GtkWidget::interior-focus = 1