Add Figure Enter/Leave Events to Webagg

[blink1073]

To test:

import matplotlib
import itertools
import numpy as np
matplotlib.use('webagg')
import matplotlib.pyplot as plt
plt.close('all')
fig, ax = plt.subplots()
x = np.linspace(0,10,100)
y = np.sin(x)
ln, = ax.plot(x,y, 'o', picker=5)
evt = []
colors = iter(itertools.cycle(['r', 'g', 'b', 'k', 'c']))
def on_event(event):
    evt.append(event)
    ln.set_color(next(colors))
    fig.canvas.draw()
    fig.canvas.draw_idle()
fig.canvas.mpl_connect('figure_enter_event', on_event)
fig.canvas.mpl_connect('figure_leave_event', on_event)
plt.show()

[tacaswell]

backported as 771664f