Fix disconnecting events when closing figure and remove_background is active

[ericpre]

Progress of the PR

• Fix disconnecting events when closing figure and remove_background is active
• add tests,
• ready for review.

Minimal example of the bug fix or the new feature

import hyperspy.api as hs
import matplotlib.pyplot as plt
import numpy as np

s = hs.signals.Signal1D(np.arange(1000).reshape((10, 100)))
s.plot()
number_connected_events = len(s.axes_manager.events.indices_changed.connected)
s.remove_background(background_type='Polynomial')
plt.close()
s.plot()

# check the events connected to `axes_manager` when calling `remove_background` are disconnected when closing the figure plot
assert number_connected_events == len(s.axes_manager.events.indices_changed.connected)

[codecov]

Codecov Report

Merging #2734 (9c54568) into RELEASE_next_patch (74c4cd4) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@                  Coverage Diff                   @@
##           RELEASE_next_patch    #2734      +/-   ##
======================================================
+ Coverage               77.24%   77.30%   +0.06%     
======================================================
  Files                     202      202              
  Lines                   30035    30037       +2     
  Branches                 6559     6560       +1     
======================================================
+ Hits                    23201    23221      +20     
+ Misses                   5086     5068      -18     
  Partials                 1748     1748              

Impacted Files	Coverage Δ
hyperspy/signal_tools.py	68.28% <100.00%> (+1.26%)	⬆️
hyperspy/drawing/signal1d.py	77.40% <0.00%> (+0.90%)	⬆️

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