UI: Add Start Streaming settings check on start

When we use the server error to respond to user's failing to start a
stream it is slow to return an error and unclear to the user what went
wrong. This diff introduces a simple settings check before attempting
to stream and catch and explain specific URL/key issues to the user.

In the case of preset services we check there is a Stream Key before
attempting to start a stream.

In the case of "custom" we only verify there is a URL since for some
services that's all that is required or the user may use user/password
authentication.

UI/window-basic-main.cpp

@@ -5754,6 +5754,20 @@ void OBSBasic::on_streamButton_clicked()
 			return;
 		}
 
+		auto action =
+			UIValidation::StreamSettingsConfirmation(this, service);
+		switch (action) {
+		case StreamSettingsAction::ContinueStream:
+			break;
+		case StreamSettingsAction::OpenSettings:
+			on_action_Settings_triggered();
+			ui->streamButton->setChecked(false);
+			return;
+		case StreamSettingsAction::Cancel:
+			ui->streamButton->setChecked(false);
+			return;
+		}
+
 		bool confirm = config_get_bool(GetGlobalConfig(), "BasicWindow",
 					       "WarnBeforeStartingStream");