Change play stop to play pause symbols and test midi support to transport button

[Jee-Bee]

Change play stop to play pause symbols and test midi support to transport button and add virtual void to assign midi control to transport at least i hope so... Maybe it helps for issue : https://github.com/openAVproductions/openAV-Luppp/issues/292

[Spengreb]

Hi Jee-Bee, I have reviewed your change and it doesnt add the midi support for transport button, it seems like more changes need to be done here.

The change to show the Pause play button is really nice however when the application first launches it still says "Stop" to fix this change this line: https://github.com/openAVproductions/openAV-Luppp/blob/master/src/gmastertrack.cxx#L203

@harryhaaren This PR is 4 years old, i think we should either close this and open a new PR or just merge it and update the above change in a subsequent PR. I dont think this closes #292 though