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:

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.