keyboard: Unset Idle ID before removing it

When toggling keyboard various times, we might get an error because trying to
disconnect an idle that isn't set anymore.

This is because when we remove the idle, we don't unset the ID.
Also clear the idle when destroying the keyboard.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/540

js/ui/keyboard.js

@@ -1144,6 +1144,7 @@ var Keyboard = class Keyboard {
             this._keyboardController.disconnect(this._keypadVisibleId);
         if (this._focusNotifyId)
             global.stage.disconnect(this._focusNotifyId);
+        this._clearShowIdle();
         this._keyboard = null;
         this.actor.destroy();
         this.actor = null;
@@ -1229,6 +1230,7 @@ var Keyboard = class Keyboard {
         if (!this._showIdleId) {
           this._showIdleId = GLib.idle_add(GLib.PRIORITY_DEFAULT_IDLE, () => {
               this.show(Main.layoutManager.focusIndex);
+              this._showIdleId = 0;
               return GLib.SOURCE_REMOVE;
           });
           GLib.Source.set_name_by_id(this._showIdleId, '[gnome-shell] this.show');