Canvas controls can now draw their own focus indicators

Canvas controls will gain an AllowFocusRing As Boolean property so custom Canvas-based controls can accept keyboard focus without also displaying Objo's standard focus ring. The property will default to True; setting it to False will suppress only the standard visual ring while preserving keyboard focus, tab-order participation, key events, and FocusReceived/FocusLost events. This lets developers draw a focus indicator appropriate to their custom control without showing two competing indicators.