- Move ModelRefreshed signal to outer class (now a QObject subclass)
- Move mimeData() to proxy model, where it needs to be for views
- (This allowed dropping the inner Files and Transitions model classes
  entirely, replaced with a stock `QStandardItemModel`. All of the
  special logic is in the custom proxy model class.
- Add a persistent QItemSelectionModel to all three models, shared by
  both views. Selection is no longer lost when switching view.
- Track the currently visible view for 'foo' (either fooListView or
  fooTreeView) in main_window, access as `get_app().window.fooView`

- Selection is now managed completely in the Files model (outer class),
  directly from the persistent QItemSelectionModel.
- Helper functions in both files_model and main_window ease transition
- updateSelection() is also gone, as there's no selection cache.
  Selection data always comes live from the selection model.
- Also greatly simplify drag-and-drop code, using proxy & selection
  models

Major overhaul to the code for adding items to the file model.
- `add_file()` is gone from both views
- files_model's new `add_files()` (<= note the plural) takes a list of
  paths, instead of churning over them one at a time
- Image sequence management is also consolidated into files_model
- Two helper methods in main_window handle communication with the user
  (in the form of popup questions / message boxes) for `add_files()`

Also, QPersistentModelIndex()es are stored in the custom model outer
classes' in-memory hashes, and can be used to locate a given item's
row for later modification (or deletion, specifically). No more
having to iterate over all the items to find one! (This is done for
Transitions as well, though it's currently not used there.)

- properties_tabelview will pull data directly from files_model and
  transition_model on the get_app().window object.
- A connection to both models' ModelRefreshed signals that forces
  a menu data reset replaces the previous update_model() calls

- When passed, OpenShot will attempt to load QAbstractItemModelTester
  into the files, emoji, effects, and transition models (base and all
  proxy models). This requires Qt 5.11 or higher.
- By default, if the environment variable `QT_LOGGING_RULES` is not
  set, it will be set to "qt.modeltest.debug=true", which causes quite
  a lot of console spew. It can be set to something less verbose before
  launching OpenShot, e.g. "qt.modeltest.warning=true".

- Switch from TitleFileName-N for numbered filenames, to Windows-esque
  "TitleFileName (N)", because the previous value would trigger the
  image sequence import logic