Allow hiding signals provided by extensions

[jlaehne]

Allow setting optional "hidden" key in definition of signals extending hyperspy, which prevents them from being shown by print_known_signal_types().

Progress of the PR

• Change implemented (can be split into several points),
• update user guide (if appropriate),
• add an changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting changelog entry in the readthedocs doc build of this PR (link in github checks)
• [n/a] add tests,
• ready for review.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (b104055) 80.23% compared to head (6da0c52) 80.22%.

Files	Patch %	Lines
hyperspy/utils/__init__.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           RELEASE_next_patch    #3302      +/-   ##
======================================================
- Coverage               80.23%   80.22%   -0.02%     
======================================================
  Files                     147      147              
  Lines                   21846    21846              
  Branches                 5140     5140              
======================================================
- Hits                    17528    17525       -3     
- Misses                   3112     3116       +4     
+ Partials                 1206     1205       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.