[FIX] OWTreeGraph: Update node text when selecting target class

[VesnaT]

Issue

To reproduce: File (iris) -> Classification Tree -> Tree Viewer -> Select i.e. Iris-setosa (Target class combo) -> nodes texts remain unchanged (only color changes).

Description of changes

Change nodes text after Target class has changed

Includes

• Code changes
• Tests
• Documentation

[astaric]

I have restarted the travis build, as its result did not make it to github (the tests passed).

[codecov-io]

Codecov Report

Merging #2045 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2045      +/-   ##
==========================================
+ Coverage   70.72%   70.75%   +0.02%     
==========================================
  Files         343      343              
  Lines       54485    54493       +8     
==========================================
+ Hits        38534    38555      +21     
+ Misses      15951    15938      -13

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