Fix cursor when drag-dropping in MML

[pavelbraginskiy]

Previously I implemented the ability to drag files onto MML using DropTarget. This worked, but it caused various cosmetic problems, most notably where the mouse cursor would often be wrong when dragging an item, showing the Forbidden cursor when a drop actually was possible.

This moves the code from DropTarget to TransferHandler. TransferHandler is more capable than DropTarget and works much better here.

Other than moving the code from DropTarget to TransferHandler, this PR doesn't actually change any code except to show the user an error message when they try to drag an invalid file onto MML.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 2.16%. Comparing base (7e572e1) to head (b99df8a).
Report is 6 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##             master   #1657      +/-   ##
===========================================
- Coverage      2.16%   2.16%   -0.01%     
  Complexity      209     209              
===========================================
  Files           268     268              
  Lines         30924   30927       +3     
  Branches       5290    5290              
===========================================
  Hits            669     669              
- Misses        30098   30101       +3     
  Partials        157     157              

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