Fix JTemplateTree bugs

[FWDekker]

Fixes #485:

• Drag-and-drop behaviour has been added back in by no longer assuming that exchangeRows always takes consecutive rows. For ease of use (and more precision) the code relies on RefinedDropSupport, but unfortunately this means that the model is now again bound to the view, but luckily that is limited to two fields. These changes have been well-documented and well-tested.
• The issues with using arrow keys seems to have been resolved by allowing the model to map view-based indices to model-based indices in a few select cases. This, too, is explained better in the code's docs than here.
• Duplication issues were resolved by checking upon insertion of a node that this causes no conflicts with existing schemes, and automatically resolving these conflicts.

[codecov]

Codecov Report

Merging #490 (9a3338f) into main (dbdde6c) will increase coverage by 0.00%.
The diff coverage is 94.11%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #490   +/-   ##
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  Coverage   85.92%   85.92%           
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  Files          51       51           
  Lines        1897     1940   +43     
  Branches      321      333   +12     
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+ Hits         1630     1667   +37     
- Misses        209      213    +4     
- Partials       58       60    +2