Fix concatenation of timed GUI commands

In idWindow::AddCommand() if a command already exists, it will add " ; "
and then the additional command. This works.

idWindow::Time() also  does a concatenation for commands
on gui->GetPendingCmd(), but so far did did not add anything in between
the commands. If that actually happened, an invalid command string would
be used later. To fix this I also add " ; " between the commands now.
Also did some little tweaks to only do this if actually adding a command

This bug became obvious in the Hexen: Edge Of Chaos ("eoc") mod, when
returning to the main menu sometimes you'd get
"WARNING: Couldn't load sound 'hexen_mainmenuplay.wav' using default"
because the commands "music hexen_mainmenu" and "play hexen_fire_menu"
got concatenated.

Author: DanielGibson

neo/ui/Window.cpp

@@ -1114,8 +1114,13 @@ void idWindow::Time() {
 			}
 		}
 	}
-	if ( gui->Active() ) {
-		gui->GetPendingCmd() += cmd;
+	if ( gui->Active() && cmd.Length() > 0 ) {
+		// DG: can't just append the command, must separate commands with " ; "
+		idStr& pend = gui->GetPendingCmd();
+		if ( pend.Length() > 0 ) {
+			pend += " ; ";
+		}
+		pend += cmd;
 	}
 }