[ENH] Save widgets: Store paths relative to workflow directory

[janezd]

Issue

Fixes part of #4464 by storing paths relative to the workflow directory.

Description of changes

Path is always relative to the workflow directory, and the saved file must be either in the same director or in its subtree. On loading the widget, auto-save is disabled unless a relative path is used to prevent auto saving to arbitrary locations when the workflow is loaded.

Implementation does not use RecentPath because it does not fit.

All core widgets for saving (Save, Save Distances, Save Model) are derived from this base class, so the patch should work for them, as well as for all properly written widgets in add-ons (Save Network).

I took care to keep the backward compatibility, though ... please check all potentially affected add-ons.

Unlike codecov, I believe that coverage is 100 %. At least judging by codecov's diff.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4532 into master will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4532      +/-   ##
==========================================
+ Coverage   83.38%   83.46%   +0.08%     
==========================================
  Files         279      276       -3     
  Lines       55780    55267     -513     
==========================================
- Hits        46510    46130     -380     
+ Misses       9270     9137     -133     

[PrimozGodec]

The implementation works correctly. There is only one thing I would change.

When setting a location to save data the text at the save button changes to Save as <filname> if I save data with name foo.csv the button has the text Save as foo.csv. Then I save the workflow and reopen it. I would like to see a button with the same text again (it would give a user feels that the path to save data is already set). After reopening text at the button is just Save

[PrimozGodec]

Thank you. It is great now.