OWImageGrid: added example run at the end of the file, data default input

[PrimozGodec]

Issue

Example widget run configuration for image grid at the bottom of the file was broken.

Description of changes

It is fixed and simplified now. Data input made as default.

Includes

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

[codecov[bot]]

Codecov Report

Merging #159 into master will increase coverage by 0.4%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #159     +/-   ##
=========================================
+ Coverage   61.14%   61.54%   +0.4%     
=========================================
  Files          13       13             
  Lines        3253     3230     -23     
  Branches      482      480      -2     
=========================================
- Hits         1989     1988      -1     
+ Misses       1140     1118     -22     
  Partials      124      124