[FIX] owcsvimport: Fix last/recent item serialization

[ales-erjavec]

Issue

'CSV Import' does not store the current file/options in a workflow scheme if the current file is not explicitly selected after the widget is placed in the workflow.

Steps to reproduce

1. Place the widget on the workflow and let it load the last loaded file (do not change anything), save the workflow.
2. Create a new workflow and add the 'CSV Import' widget, select a different file to load.
3. Reload the workflow from step 1. The widget will load the file from the second step.

Selecting a different file in step 1 would correctly store the item.

Description of changes

Call set_current_file in __init__ to record the initial loaded file in the scheme_only recorded _session_items.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4272 into master will increase coverage by 0.25%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4272      +/-   ##
==========================================
+ Coverage   86.22%   86.47%   +0.25%     
==========================================
  Files         396      396              
  Lines       70706    70748      +42     
==========================================
+ Hits        60964    61181     +217     
+ Misses       9742     9567     -175