[FIX] plotutils: Fix scene layout tracking in AnchorItem

[ales-erjavec]

Issue

Fixes #6854

Since 384f6f4 the AnchorItem does not properly update its layout due to difference in pg.GraphicsObject/pg.GraphicsWidget implementations.

Description of changes

Track scene transform changes.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.27%. Comparing base (70ebf27) to head (c275a4a).
Report is 184 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6859   +/-   ##
=======================================
  Coverage   88.27%   88.27%           
=======================================
  Files         326      326           
  Lines       71137    71143    +6     
=======================================
+ Hits        62793    62801    +8     
+ Misses       8344     8342    -2