pgui: allow keyboard keys to be added as PGButton click buttons

These will respond as clicks not when the mouse cursor is hovering over them, but when they have keyboard focus.

Fixes #600

panda/src/pgui/pgButton.cxx

@@ -115,7 +115,11 @@ release(const MouseWatcherParameter &param, bool background) {
   if (has_click_button(param.get_button())) {
     _button_down = false;
     if (get_active()) {
-      if (param.is_outside()) {
+      // Note that a "click" may come from a keyboard button press.  In that
+      // case, instead of checking that the mouse cursor is still over the
+      // button, we check whether the item has keyboard focus.
+      if (param.is_outside() &&
+          (MouseButton::is_mouse_button(param.get_button()) || !get_focus())) {
         set_state(S_ready);
       } else {
         set_state(S_rollover);