see prev commit.
boardEngine default is -1, not 0.
Rework Arrow Options a bit and add an 's' key binding for displaying Engine PV on mainboard - with code from Patrick. Todo - resolve ::mainEngine and analysis(boardEngine).
Old xboard housekeeping. Rename sendMoveToEngine to sendMoveToXboardEngine.
Patrick's feature to throttle the engine arrow redraws to 200ms, and some hardening to updateAnalysisText
Related analysis board/help tweaks
Makefile.macOS mac_gui tweak.
Make a specific Options->Arrows option menu, and also a feature to colourise PV arrows. Also, a bugfix for tb menu state, and stray ')' in analysis.tcl. With code from Patrick/GLM-Flash-AI