UI: Add option to hide OBS windows on Windows

This uses the SetWindowDisplayAffinity API to hide windows from capture
applications (including OBS). This is not perfect - internal windows
such as context menus, combo box dropdowns, etc will still be displayed.
Even with these limitations, it should help people with single monitors
capture content with less interference from the OBS window.

This implementation is for Windows only but the code is generic enough
that adding other platforms should be straightforward.

UI/window-projector.cpp

@@ -30,6 +30,10 @@ OBSProjector::OBSProjector(QWidget *widget, obs_source_t *source_, int monitor,
 	if (isAlwaysOnTop)
 		setWindowFlags(Qt::WindowStaysOnTopHint);
 
+	// Mark the window as a projector so SetDisplayAffinity
+	// can skip it
+	windowHandle()->setProperty("isOBSProjectorWindow", true);
+
 #if defined(__linux__) || defined(__FreeBSD__) || defined(__DragonFly__)
 	// Prevents resizing of projector windows
 	setAttribute(Qt::WA_PaintOnScreen, false);