Update use of markers to HyperSpy v2

[hakonanes]

Description of the change

The goal of this PR is to update our markers to use the new API in HyperSpy v2.

Our API should remain the same will remain mostly the same. Look-up of markers added to an EBSD signal changed in HyperSpy v2, though. Other changes:

• Zone axes labels when plotted as signal markers won't have a contrasting (white) background (Text markers in v2 does not support bbox keyword argument hyperspy/hyperspy#3458)

This work was started in #651 but was never completed.

Progress of the PR

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean code style by running black via pre-commit
• All tests pass
• All docs build as before

Minimal example of the bug fix or new feature

>>> import kikuchipy as kp
>>> s = kp.data.nickel_ebsd_large()
>>> vbse_imager = kp.imaging.VirtualBSEImager(s)
>>> p = vbse_imager.plot_grid()
>>> p
<EBSD, title: patterns Scan 1, dimensions: (|60, 60)>
>>> p.metadata.Markers
├── HorizontalLines = <HorizontalLines (HorizontalLines), length: 6>
├── Texts = <Texts (Texts), length: 25>
└── VerticalLines = <VerticalLines (VerticalLines), length: 6>

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 CHANGELOG.rst.
• New contributors are added to kikuchipy/__init__.py and .zenodo.json.

[CSSFrancis]

@hakonanes I can try to take a bit of a closer look but I guess I don't understand why you have so many small text collections rather than one big text collection? It is faster to have one I think.

[hakonanes]

The old top comment's code snippet didn't reflect the signal markers; it does now.

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