[FIX] canvas/annotationitem: Use separate item for shadow base

[ales-erjavec]

Issue

Arrow annotations in the canvas are rendered badly on high dpi monitors. Even more so in the 'Preview' for the 'Examples' and 'Recent' dialogs. The reason for this is that QGraphicsEffects render indirectly 'item -> pixmap -> effect -> draw pixmap'

Description of changes

Workaround for QTBUG-65035

Use a separate item for shadow base that is stacked behind the 'true' arrow geometry item.

(the same was already done for the node items in 3be3f1a)

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #2821 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #2821      +/-   ##
=========================================
- Coverage    76.2%   76.2%   -0.01%     
=========================================
  Files         337     337              
  Lines       59665   59665              
=========================================
- Hits        45470   45467       -3     
- Misses      14195   14198       +3