EditorManagerPlaceHolder: Don't restrict to one per mode

Make visibility the deciding factor which placeholder takes the main
editor area. A mode can have multiple placeholders as long as only a
single one is visible at a time.

Change-Id: I80b86dcb9e31b29f87fe42a465a3d3ad49eeef17
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>

src/plugins/debugger/debuggermainwindow.cpp

@@ -205,7 +205,7 @@ void DebuggerMainWindow::finalizeSetup()
 QWidget *createModeWindow(const Core::Id &mode, DebuggerMainWindow *mainWindow, QWidget *central)
 {
     if (!central)
-        central = new EditorManagerPlaceHolder(mode);
+        central = new EditorManagerPlaceHolder;
 
     auto editorHolderLayout = new QVBoxLayout;
     editorHolderLayout->setMargin(0);