Base widget tests: Require keywords

[janezd]

Issue

Fixes #7174.

Description of changes

• Add a test that every widget has a string with keywords.
  The test tries to not cause false positives: not having keywords does not result in crashes but only causes problems for translators, which can complain about this.

  • The test is opt-in: add-ons can -- but don't have to -- register for the test.
  • The widget is tested only if it is final class and contains a name.
  • Despite this, mock widgets have to define keywords if they define a name.

This PR also adds keywords where necessary.

Includes

• Code changes
• Tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.03%. Comparing base (9e35194) to head (4cf0bd5).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7218   +/-   ##
=======================================
  Coverage   89.03%   89.03%           
=======================================
  Files         335      335           
  Lines       74243    74253   +10     
=======================================
+ Hits        66100    66112   +12     
+ Misses       8143     8141    -2     

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