Restore toolbar for tiled plots

[nvaytet]

Fixes 2 cases:

1. When making tiled plots using the %matplotlib widget backend, the toolbar would disappear.

import plopp as pp
%matplotlib widget

f = pp.data.data1d().plot()
f / f

2. Toolbar would also be absent when making figures using custom axes, e.g.:

import plopp as pp
import matplotlib.pyplot as plt
%matplotlib widget

fig, ax = plt.subplots(2, 1)
f1 = pp.data.data1d().plot(ax=ax[0])
f2 = pp.data.data2d().plot(ax=ax[1])
fig.canvas

The fix is to not alter the visibility of the toolbar if axes come from the outside world.

Note that this does mean that if you do something like

fig, ax = plt.subplots(2, 1)
f = pp.data.data1d().plot(ax=ax[0])
f

you will see 'double' toolbar, but usually if one supplies the ax argument, it implies that we want to use the matplotlib figure instead of the Plopp figure.

[nvaytet]

Changing to draft as #378 needs to be merged first.