[FIX] schemeedit: Clear edit focus before removing items

[ales-erjavec]

Issue

When the canvas text annotation is selected and has edit focus, pressing (Control|Command) + Backspace raises an error

----------------------------- KeyError Exception ------------------------------
Traceback (most recent call last):
  File "/Users/aleserjavec/workspace/orange3/Orange/canvas/document/schemeedit.py", line 1339, in __onEditingFinished
    annot = self.__scene.annotation_for_item(item)
  File "/Users/aleserjavec/workspace/orange3/Orange/canvas/canvas/scene.py", line 618, in annotation_for_item
    return rev[item]
KeyError: <Orange.canvas.canvas.items.annotationitem.TextAnnotation object at 0x1246ee318>
-------------------------------------------------------------------------------

Description of changes

Clear item's keyboard focus before removing it.

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #2427 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #2427     +/-   ##
=========================================
- Coverage   74.31%   74.21%   -0.1%     
=========================================
  Files         320      320             
  Lines       55799    55799             
=========================================
- Hits        41468    41413     -55     
- Misses      14331    14386     +55