Add drag and drop image folder functionality

[mfl28]

• Adds feature that allows to drag and drop a folder (from outside the ap)p into the app and automatically load the images contained.

Closes #38 .

[codecov]

Codecov Report

Base: 85.60% // Head: 85.61% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (ee3ec40) compared to base (550a1fa).
Patch coverage: 92.30% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #39      +/-   ##
============================================
+ Coverage     85.60%   85.61%   +0.01%     
- Complexity     1363     1368       +5     
============================================
  Files            91       91              
  Lines          5626     5639      +13     
  Branches        509      511       +2     
============================================
+ Hits           4816     4828      +12     
  Misses          548      548              
- Partials        262      263       +1     

Impacted Files	Coverage Δ
...om/github/mfl28/boundingboxeditor/ui/MainView.java	83.06% <92.30%> (+0.51%)	⬆️

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

95.2% Coverage
0.0% Duplication