Table: Add 'Clear Selection', disable 'Restore order' when order is original

[janezd]

Issue

Closes #6969.

Description of changes

I think we agreed (in the core group meeting) that the title of the box stays Variables, because it is about variables. We also said that the behaviour of the output button is consistent with other widgets in Orange and that selection behaviour is consistent with common GUIs.

I thus fixed the second point, disabling of reorder button.

As a bonus, I added a button to clear selection (which is also disabled when there's no selection). We have enough space, but its placement doesn't look nice. Perhaps move it to next to "Restore Original Order"?

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.77%. Comparing base (5eb97b8) to head (245e3ca).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7125   +/-   ##
=======================================
  Coverage   88.77%   88.77%           
=======================================
  Files         334      334           
  Lines       73671    73681   +10     
=======================================
+ Hits        65402    65412   +10     
  Misses       8269     8269           

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