Improve snapping behavior and add undo in EditableTemplate

[philippjfr]

Previously there was a bug in the snapping behavior which meant that often it would just snap to itself or an invisible element. This fixes both cases and also adds an undo button to the dashboard builder.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 70.82%. Comparing base (5f77d4f) to head (27334cf).
Report is 1 commits behind head on main.

❗ Current head 27334cf differs from pull request most recent head 79dd353. Consider uploading reports for the commit 79dd353 to get more accurate results

Files	Patch %	Lines
panel/io/handlers.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6687      +/-   ##
==========================================
- Coverage   71.08%   70.82%   -0.26%     
==========================================
  Files         313      305       -8     
  Lines       46177    45985     -192     
==========================================
- Hits        32824    32570     -254     
- Misses      13353    13415      +62     

Flag	Coverage Δ
unitexamples-tests	70.82% <0.00%> (-0.26%)	⬇️

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