feat: Add .lm_dragging to .lm_root and style nested dragging, also style default empty dashboard

[dsmmcken]

For upcoming support of nested ui.dashboards. Gives a visual boundry of the nested layout, showing that children can't be dragged out of the nested layout.

Relates to: deephaven/deephaven-plugins#1238

[codecov]

Codecov Report

❌ Patch coverage is 9.09091% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.36%. Comparing base (4d7c106) to head (8eef6c3).

Files with missing lines	Patch %	Lines
packages/golden-layout/src/utils/DragListener.ts	0.00%	5 Missing ⚠️
.../golden-layout/src/controls/DragSourceFromEvent.ts	0.00%	3 Missing ⚠️
packages/golden-layout/src/controls/DragSource.ts	0.00%	1 Missing ⚠️
packages/golden-layout/src/controls/Tab.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2560      +/-   ##
==========================================
+ Coverage   44.11%   44.36%   +0.25%     
==========================================
  Files         764      764              
  Lines       43104    43110       +6     
  Branches    10895    10897       +2     
==========================================
+ Hits        19014    19125     +111     
+ Misses      24074    23969     -105     
  Partials       16       16              

Flag	Coverage Δ
unit	44.36% <9.09%> (+0.25%)	⬆️

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