Selectors improvement: rotation, aspect ratio correction and add/remove state

The RectangleSelector and EllipseSelector can now be rotated interactively between -45° and 45°. The range limits are currently dictated by the implementation. The rotation is enabled or disabled by striking the r key ('r' is the default key mapped to 'rotate' in state_modifier_keys) or by calling selector.add_state('rotate').

The aspect ratio of the axes can now be taken into account when using the "square" state. This is enabled by specifying use_data_coordinates='True' when the selector is initialized.

In addition to changing selector state interactively using the modifier keys defined in state_modifier_keys, the selector state can now be changed programmatically using the add_state and remove_state methods.

import matplotlib.pyplot as plt
from matplotlib.widgets import RectangleSelector
import numpy as np

values = np.arange(0, 100)

fig = plt.figure()
ax = fig.add_subplot()
ax.plot(values, values)

selector = RectangleSelector(ax, print, interactive=True,
selector.add_state('rotate') # alternatively press 'r' key
# rotate the selector interactively

selector.remove_state('rotate') # alternatively press 'r' key