Fix list layout creation for unregistered sendable types

[Gold872]

• Allows sendables with an unknown/unregistered type to be turned into a list layout
• Speeds up creation of list layouts
• Fixes an issue with list layouts not unsubscribing if a drag finishes before list layout is created

Resolves #141

[codecov]

Codecov Report

Attention: Patch coverage is 77.35849% with 12 lines in your changes missing coverage. Please review.

Project coverage is 66.97%. Comparing base (1a993f3) to head (8aa39be).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...b/widgets/network_tree/networktables_tree_row.dart	57.14%	6 Missing ⚠️
lib/widgets/network_tree/networktables_tree.dart	84.37%	5 Missing ⚠️
lib/services/nt_widget_builder.dart	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #151      +/-   ##
==========================================
+ Coverage   66.67%   66.97%   +0.29%     
==========================================
  Files          78       78              
  Lines        8085     8114      +29     
==========================================
+ Hits         5391     5434      +43     
+ Misses       2694     2680      -14     

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