Add Support for NSIS plugins in Windows installer

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

Improve handling of frozen resource files

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.

v0.1.2

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'
])

v0.1.1

Improve placement of icons in Mac installer.

v0.1.0

Make it possible to customize the NSIS installer on Windows and fix FileNotFoundError during fbs freeze on Windows.

v0.0.9

Improve handling of missing UCRT DLLs on Windows. See mherrmann/fbs-tutorial#4.

v0.0.8

Fix error when installing via pip on Ubuntu

v0.0.7

Set version number for release 0.0.7

v0.0.6

Set version number for release 0.0.6

v0.0.5

Set version number for release 0.0.5