[ENH] Allow add-ons to register file format for the Save widget

[janezd]

Issue

Resolves #3701.

Description of changes

The first commit replaces class attribute filters of OWSaveBase with a class method get_filters to allow derived classes to create the list dynamically. Class attribute filters still exists and the provided get_filters returns it; this is to ensure the backward compatibility. Making filters a property instead of creating a method is not a good alternative: filters is used in some class methods that cannot be easily changed into ordinary methods (i.e. methods of classes that implement platform-specific dialogs). Thus filter would have to be a class property, but class properties are messy hacks (unless we want to have a meta class for OWSaveBase).

The second commit implements get_methods for OWSave.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4302 into master will decrease coverage by <.01%.
The diff coverage is 97.95%.

@@            Coverage Diff             @@
##           master    #4302      +/-   ##
==========================================
- Coverage   86.69%   86.69%   -0.01%     
==========================================
  Files         396      396              
  Lines       71561    71577      +16     
==========================================
+ Hits        62039    62052      +13     
- Misses       9522     9525       +3