Refactor key-combinations into separate class

[mfl28]

• Move key-combinations from inner Controller class into separate KeyCombinations class.

Closes #135.

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.48%. Comparing base (3cc364b) to head (62e6679).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
.../boundingboxeditor/controller/KeyCombinations.java	92.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #136   +/-   ##
=========================================
  Coverage     86.48%   86.48%           
- Complexity     1498     1499    +1     
=========================================
  Files            96       97    +1     
  Lines          5986     5986           
  Branches        549      549           
=========================================
  Hits           5177     5177           
  Misses          532      532           
  Partials        277      277           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.