Add more keypress events

[banesullivan]

Resolves #149 by adding the following nifty keyboard shortcuts:

• shift+s: Save a screenhsot (only on BackgroundPlotter) (control/command would not register)
• c: Enable interactive cell selection/picking
• up/down (arrows): Zoom in and out
• +/-: Increase/decrease the point sizes and line widths
• numerical keys are linked to saved camera positions in the BackgroundPlotter

Note that this should resolve the issues where the key bindings were not being added to the BackgroundPlotter - someone may need to double-check my work to make sure the BackgroundPlotter is still closing properly

[banesullivan]

import pyvista as pv
from pyvista import examples

mesh = examples.download_bunny_coarse()

p = pv.BackgroundPlotter()
p.add_mesh(mesh, show_edges=True)
demo = lambda *args: p.add_text("Pressed: {}".format(p.iren.GetKeySym()), name="msg")
p.iren.AddObserver("KeyPressEvent", demo)

[akaszynski]

Looks good. It's tricky getting the callbacks to play nicely as both vtk and pyqt want control of the main thread. These should work, but it's too bad we can't test them automatically.