Same plugin-manager is used in all uca-camera instances

[MarcusZuber]

Used for ufo-kit/libuca#102 . (There the plugin manager keeps track of the uca-net-sub-cameras, which require the same plugin manager instance)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.89%. Comparing base (9a84163) to head (318ad02).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #554   +/-   ##
=======================================
  Coverage   82.89%   82.89%           
=======================================
  Files         133      133           
  Lines       10933    10933           
=======================================
  Hits         9063     9063           
  Misses       1870     1870           

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[MarcusZuber]

When you ask why I didn't create the instance in the class body: Then I have to import gi.xxx globally, which we anted to prevent since we do not want to have it as hard dependency.