Add ability to remove inactive items

[Robin-St]

This is to close #396.

The find_item methods are extended with the argument only_active that defaults to true to keep the old behaviour intact. The various delete functions calls find_item with only_active=False to find any inactive items and be able to delete them.

Test cases are added with copies of the various test cases to remove an item but to try remove req5 that is inactive. Some test files used the test cases in the tutorial folder, as I could not find an inactive req there I added tut26 as an inactive case in that folder.

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (f34c454) 95.50% compared to head (987a839) 95.35%.

Files	Patch %	Lines
doorstop/core/tree.py	42.85%	4 Missing :warning:
doorstop/core/document.py	75.00%	0 Missing and 1 partial :warning:

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #619      +/-   ##
===========================================
- Coverage    95.50%   95.35%   -0.16%     
===========================================
  Files           41       41              
  Lines         5053     5059       +6     
  Branches      1184     1187       +3     
===========================================
- Hits          4826     4824       -2     
- Misses         143      150       +7     
- Partials        84       85       +1     

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