MacOS: headless UX improvements

don't show the application in the dock or task switcher unless:
- we create a non-headless display driver or
- an alert must be shown

misc/dist/macos_tools.app/Contents/Info.plist

@@ -45,6 +45,8 @@
 		<key>x86_64</key>
 		<string>10.12</string>
 	</dict>
+	<key>LSUIElement</key>
+	<true/>
 	<key>NSHighResolutionCapable</key>
 	<true/>
 	<key>CFBundleDocumentTypes</key>

platform/macos/display_server_macos.mm

@@ -3469,6 +3469,9 @@
 
 	CGEventSourceSetLocalEventsSuppressionInterval(event_source, 0.0);
 
+	// Now that we are sure we're not headless, display our icon in the dock/task switcher.
+	[NSApp setActivationPolicy:NSApplicationActivationPolicyRegular];
+
 	int screen_count = get_screen_count();
 	for (int i = 0; i < screen_count; i++) {
 		display_max_scale = fmax(display_max_scale, screen_get_scale(i));

platform/macos/os_macos.mm

@@ -177,6 +177,9 @@
 }
 
 void OS_MacOS::alert(const String &p_alert, const String &p_title) {
+	// Make sure we are shown in the dock/task switcher now (even if we were headless).
+	[NSApp setActivationPolicy:NSApplicationActivationPolicyRegular];
+
 	NSAlert *window = [[NSAlert alloc] init];
 	NSString *ns_title = [NSString stringWithUTF8String:p_title.utf8().get_data()];
 	NSString *ns_alert = [NSString stringWithUTF8String:p_alert.utf8().get_data()];
@@ -820,9 +823,6 @@
 	// Implicitly create shared NSApplication instance.
 	[GodotApplication sharedApplication];
 
-	// In case we are unbundled, make us a proper UI application.
-	[NSApp setActivationPolicy:NSApplicationActivationPolicyRegular];
-
 	// Menu bar setup must go between sharedApplication above and
 	// finishLaunching below, in order to properly emulate the behavior
 	// of NSApplicationMain.