[FIX] Identify attributes without image type as image attributes

[PrimozGodec]

Issue

• Fixes https://github.com/biolab/orange3-imageanalytics/issues/191
• Attributes are differently selected in different widgets
• Paths are differently treated accros widgets (and code is duplicated).

Description of changes

• Common module for image attribute identification and paths concatenation
• Use those common functions in the widget

Includes

• [X] Code changes
• [X] Tests
• [ ] Documentation

[codecov[bot]]

Codecov Report

Merging #207 (83ae579) into master (059d256) will increase coverage by 1.12%. The diff coverage is 94.82%.

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
+ Coverage   67.85%   68.97%   +1.12%     
==========================================
  Files          15       17       +2     
  Lines        2809     2830      +21     
  Branches      440      434       -6     
==========================================
+ Hits         1906     1952      +46     
+ Misses        788      769      -19     
+ Partials      115      109       -6