Make it possible to reoder tabs in the diagram editor

[prmr]

This is a one line change in EditorFrame:

public EditorFrame(Stage pMainStage, Optional<File> pOpenWith) 
{
    ...
    TabPane tabPane = new TabPane();
    tabPane.setTabDragPolicy(TabPane.TabDragPolicy.REORDER);

Update: Unfortunately this JavaFX feature is buggy. Tabs get stuck (impossible to open), it's impossible to close the last tab, etc. We'll revert this one for now in the hopes the feature becomes more stable.