Make plotting of window colorbar optional, return only figure

[hakonanes]

Description of the change

• Address Allow not plotting a colorbar when plotting a Window #365 by adding a colorbar=True parameter to kikuchipy.filters.Window.plot().
• Return only figure from kikuchipy.filters.Window.plot() if return_figure=True (default is False). Previous behaviour was to always return a tuple with figure, image array and colorbar. The image array and colorbar are obtainable from the figure.

Close #365.

Progress of the PR

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean code style by running black via pre-commit

Minimal example of the bug fix or new feature

>>> import kikuchipy as kp
>>> w = kp.filters.Window()
>>> w.plot(colorbar=False)
>>> fig = w.plot(return_figure=True)
>>> ax = fig.axes[0]
>>> im = ax.get_images()[0]
>>> cbar = im.colorbar

For reviewers

• The PR title is short, concise, and will make sense 1 year later.
• New functions are imported in corresponding __init__.py.
• New features, API changes, and deprecations are mentioned in the
  unreleased section in doc/changelog.rst.

[hakonanes]

Will merge when checks pass.