osx backend alias

[greglucas]

Everything seems to work well with macosx from a standard Python terminal, but I decided to switch over to ipython and noticed that there is a mismatch in the alias names for the event loops (ipython seems to look for "osx" instead of "macosx").

import mpl_gui as mg
from matplotlib.figure import Figure

fig1 = Figure(label='A Label!')

fig2 = Figure()

mg.show([fig1, fig2])

UnknownBackend: No event loop integration for 'macosx'. Supported event loops are: qt, qt4, qt5, qt6, gtk, gtk2, gtk3, gtk4, tk, wx, pyglet, glut, osx, asyncio