Moved visible property from ExitDialog.qml to the dialogs themselves in MainWindow.qml, and exposed the text of the dialog as a property so it can be modified more easily.
Replaced the openGraph dialog with an ExitDialog, removed the abortButtonClicked signal from ExitDialog.qml since it's always handled the same way (hide the ExitDialog), and set the ExitDialog to hide itself automatically when a button is clicked instead of having to do it manually when instanced.

Fixed up the interaction between the ExitDialog's and finderSaveGraph, we now pass a bool in signals from the ExitDialog's to trigger finderSaveGraph.show(). The bool indicates whether to create a new graph or quit the application.