trackbar: refactor the classes and introduced a  new TfpgBaseTrackBar class.
trackbar: refactor the classes and introduced a  new TfpgBaseTrackBar class.

This process is not complete, but it already reduced a lot of code duplication.

wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI