Remove category change contextmenu item for non-View tree cells

[mfl28]

• Removes the "Change Category" contextmenu-item for object tree cells whose corresponding cell-object is not a View instance.
• Adds test assertions.

Fixes #22 .

[codecov]

Codecov Report

Merging #23 (8a0176b) into master (1902b1b) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master      #23      +/-   ##
============================================
+ Coverage     84.05%   84.08%   +0.02%     
  Complexity     1213     1213              
============================================
  Files            86       86              
  Lines          5301     5309       +8     
  Branches        476      477       +1     
============================================
+ Hits           4456     4464       +8     
  Misses          610      610              
  Partials        235      235              

Impacted Files	Coverage Δ
...28/boundingboxeditor/ui/ObjectTreeElementCell.java	94.93% <100.00%> (+0.19%)	⬆️

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

100.0% Coverage
0.0% Duplication