tkvlc example - enhancements for Windows and macOS (#237)

Added a number of private classes to deal with Tcl/Tk/tkinter idiosyncrasies and differences on Windows and macOS.  On macOS, most tkinter calls seem delayed making it necessary to wrap those as events (using the tkinter.after* function).

Changed the anchoring of the buttons and video panel by making both separate, movable windows.  Dragging and resizing each window now works smoothly and correctly on Windows and macOS.

Added menu options to change zoom, rate and opacity.  However, the zoom and rate items are disabled on macOS due to poor behavior (still unresolved).

Author: mrJean1