Make RestartGameplay offline only

This should force the RestartGameplay button to only be able to be pressed when not in a Network session.
The way this is done is basically the best way I could come up to do it with the existing architecture.
It just checks to see if the previous screen is the right one, then goes through the motions if it is.
If a theme ends up renaming that screen or messes with the order of execution then this might break.
... then it's the themer's fault, right?

Author: poco0317

src/ScreenGameplay.cpp

@@ -1905,18 +1905,23 @@ bool ScreenGameplay::Input( const InputEventPlus &input )
 		return false;
 	}
 
-	/* Restart gameplay button moved from theme to allow for rebinding for people who
-	*  dont want to edit lua files :)
-	*/
-	bool bHoldingRestart = false;
-	if (GAMESTATE->GetCurrentStyle(input.pn)->GameInputToColumn(input.GameI) == Column_Invalid)
+	// RestartGameplay may only be pressed when in Singleplayer.
+	// Clever theming or something can probably break this, but we should at least try.
+	if (SCREENMAN->GetTopScreen()->GetPrevScreen() == "ScreenSelectMusic")
 	{
-		bHoldingRestart |= input.MenuI == GAME_BUTTON_RESTART;
-	}
-	if (bHoldingRestart)
-	{
-		SCREENMAN->GetTopScreen()->SetPrevScreenName("ScreenStageInformation");
-		BeginBackingOutFromGameplay();
+		/* Restart gameplay button moved from theme to allow for rebinding for people who
+		*  dont want to edit lua files :)
+		*/
+		bool bHoldingRestart = false;
+		if (GAMESTATE->GetCurrentStyle(input.pn)->GameInputToColumn(input.GameI) == Column_Invalid)
+		{
+			bHoldingRestart |= input.MenuI == GAME_BUTTON_RESTART;
+		}
+		if (bHoldingRestart)
+		{
+			SCREENMAN->GetTopScreen()->SetPrevScreenName("ScreenStageInformation");
+			BeginBackingOutFromGameplay();
+		}
 	}