Add bounding shape image saving feature

[mfl28]

• Adds feature that allows to save bounding-shape cropped regions of images to disk (as png image files - masked-out parts of an image are transparent).
• Saving is triggered via the new Save as Image... bounding shape context-menu item (opens a file-chooser).
• Fixes an issue with key-releases not being triggered correctly.
• Updates and extends unit tests.

Closes #98.
Fixes #99.
Fixes #101 .

[codecov]

Codecov Report

Attention: 31 lines in your changes are missing coverage. Please review.

Comparison is base (ce3a8f9) 86.26% compared to head (4f46b86) 86.37%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #100      +/-   ##
============================================
+ Coverage     86.26%   86.37%   +0.11%     
- Complexity     1437     1462      +25     
============================================
  Files            92       92              
  Lines          5789     5880      +91     
  Branches        524      538      +14     
============================================
+ Hits           4994     5079      +85     
+ Misses          539      536       -3     
- Partials        256      265       +9     

Files	Coverage Δ
...l28/boundingboxeditor/ui/EditorsSplitPaneView.java	98.34% <100.00%> (+0.21%)	⬆️
...28/boundingboxeditor/ui/ImageFileExplorerView.java	100.00% <100.00%> (ø)
.../mfl28/boundingboxeditor/ui/TagScrollPaneView.java	98.85% <100.00%> (+0.11%)	⬆️
...om/github/mfl28/boundingboxeditor/ui/MainView.java	83.52% <71.42%> (+0.32%)	⬆️
...mfl28/boundingboxeditor/controller/Controller.java	78.41% <74.07%> (-0.55%)	⬇️
...28/boundingboxeditor/ui/ObjectTreeElementCell.java	95.43% <75.75%> (+0.21%)	⬆️
...8/boundingboxeditor/ui/WorkspaceSplitPaneView.java	81.90% <77.04%> (+0.08%)	⬆️

... 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

80.2% Coverage
0.0% Duplication