GH-338, GH-513, GH-700 Unify edit instance with console window

* The resulting instance window can be closed at any point.
* Main window is kept open and running instances are marked with a badge.
* Multiple instances can now run from the same MultiMC - it's even more **multi** now.
* MultiMC can be entirely closed, keeping Minecraft(s) running.

application/SettingsUI.h

@@ -23,6 +23,4 @@ void ShowPageDialog(T raw_provider, QWidget * parent, QString open_page = QStrin
 		dlg.exec();
 	}
 }
-
-void ShowInstancePageDialog(InstancePtr instance, QWidget * parent, QString open_page = QString());
 }