IGraphics context doesn't know when the window loses focus

[olilarkin]

When the user clicks away from the plug-in window, there should be a way for the IGraphics context to e.g. collapse the IPopupMenuControl, or cancel in progress edit of the ITextEntryControl. There should also be a callback so that any custom behaviour can be implemented.

IMPORTANT DETAILS

• What plug-in format does it relate to - ALL
• What platform does it relate to - Windows/macOS & ??
• What IGRAPHICS_BACKEND does it relate to - ALL

Additional context https://iplug2.discourse.group/t/how-to-catch-the-plugin-window-focus-loss-event-in-mac-os/920