Improve Macos shortcut handling

[mfl28]

• Uses platform specific shortcut (Ctrl on Windows/Linux, Cmnd on Macos) everywhere.
• Updates unit tests to work on Macos.
• Upgrades to latest JavaFX version which fixes some errors on MacOS (e.g. exception when dragging multiple files into the program).
• Upgrades to latest testFX version.

Closes #94 .

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (a08e876) 86.05% compared to head (931bffa) 86.30%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #95      +/-   ##
============================================
+ Coverage     86.05%   86.30%   +0.24%     
- Complexity     1427     1437      +10     
============================================
  Files            92       92              
  Lines          5788     5789       +1     
  Branches        524      524              
============================================
+ Hits           4981     4996      +15     
+ Misses          546      538       -8     
+ Partials        261      255       -6     

Files	Coverage Δ
.../mfl28/boundingboxeditor/ui/TagScrollPaneView.java	98.73% <100.00%> (ø)
...ndingboxeditor/ui/settings/SettingsDialogView.java	100.00% <100.00%> (ø)
...mfl28/boundingboxeditor/controller/Controller.java	78.96% <83.33%> (+0.63%)	⬆️
...fl28/boundingboxeditor/ui/BoundingPolygonView.java	85.48% <0.00%> (ø)
...fl28/boundingboxeditor/ui/EditorImagePaneView.java	88.97% <50.00%> (+2.36%)	⬆️
...ub/mfl28/boundingboxeditor/ui/BoundingBoxView.java	67.83% <0.00%> (ø)

... and 2 files with indirect coverage changes

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

86.5% Coverage
0.0% Duplication