0.51-1: update to tcl/tk 8.6

update to Pd-0.51-1: Try to update to tk 8.6

edit September 1st, 2020: added canvas_text_cursor color (only in app, get source from "next" branch)
edit September 4th, 2020: added missing canvas_text_cursor entry in pd_colors
edit September 14th, 2020: added windows installer & windows files (binary & tree) for if the installer fails