Ensure modifier change is forwarded after the key sending to input method.

As a subsequence of d66be7b9, originally I used signal slot to trigger modifier change, but this is actually wrong in the sense of event order expected to see by the key grab object.

Same as real hardware wl_keyboard, key should be sent before modifier change. For example, Left Ctrl press and release should produce key events in the order of Control_L and Control+Control_L.