Fbs

In particular, this lets us fix 24. Some added code for this is in fbs-tutorial.

Previously, the folder `src/main/resources/mac` was meant to contain a
subfolder structure that mimics the layout of the frozen .app folder.
For example: `Contents/Info.plist`. If you wanted to add a "normal"
resource file, you had to place it in `.../mac/Contents/Resources` - but
then fbs's `get_resource(...)` function was unable to find it (because it
expected the file to be at just `.../mac`, as for the other OSs).

Now, there are "normal" resources and those that are only required for
the frozen version of an app. The former go into `src/main/resources/mac`
`.../windows` etc., whereas the latter go into `.../mac-frozen` et al.

Make it possible to supply extra arguments to PyInstaller on Mac and Linux. (On Windows, it already was possible.) This lets you for instance add missing imports:

freeze_linux(extra_pyinstaller_args=[
'--hidden-import', 'pgi.overrides.GObject',
'--hidden-import', 'pgi.overrides.GLib'
])

Improve placement of icons in Mac installer.