Fix assertion when using MDLFile without GUI comps (e.g. in Unit tests)

Move the code for adding and removing propertiesWindow to the list of
ValueTree::Listeners from MDLFile to MainAppWindow to prevent ListenerList
assertion in MDLFile when not using the GUI, for example when running the
unit tests.
Reset PorpertiesWindow after MDL was closed
Set focus on the PropertiesWindow when there is no selection
Minor clean-ups
Update juce to 7fb2533