open_filename_dialog: Allow reading *.*

[janezd]

Issue

open_filename_dialog has an argument add_all which, if True, adds "All readable files" that allows loading a file with any known extension. This is not enough to implement biolab/orange3#6792.

Description of changes

• If add_all is set to * it offers all extensions (.), not just the known.
• An unrelated improvement: if there is only a single file format, "All readable files" makes no sense and is not shown. This is situation might be common - there may be only a single file format and the caller forgets to change the default value of add_all from True to False.

• Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.79%. Comparing base (9ae212f) to head (7c6f189).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #278   +/-   ##
=======================================
  Coverage   71.79%   71.79%           
=======================================
  Files          34       34           
  Lines        9114     9117    +3     
=======================================
+ Hits         6543     6546    +3     
  Misses       2571     2571           

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