WPushButton features

* Allow to connect Pushbutton to a separate Indicator control
* Introduced a GuiTick class as "unprecise" GUI Timer synced with the audio callback but without extra priority. This can be used for all GUI Timer events, where a Qt Timer event is to uncooperative.
* Introduced "play_indicator" CO for CDJ like blinking play button.  
* Introduced a "Long Press Latching" toggle button, which can be used for "beatsync" control in master sync branch.
* Removed <RightClickIsPushButton> because it is not needed.

I have also prepared a test Skin in https://github.com/daschuer/mixxx/tree/cdj_skins