Layouteditor updates

[raphaelquast]

This PR implements some improvements for the LayoutEditor:

• add background image capabilities (e.g. show backgrounds during resize/reposition)
• add option to include map extent in layout editor

[codecov]

Codecov Report

❌ Patch coverage is 75.96154% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.73%. Comparing base (3313346) to head (da74de1).
⚠️ Report is 475 commits behind head on dev.

Files with missing lines	Patch %	Lines
eomaps/helpers.py	75.96%	25 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #199      +/-   ##
==========================================
+ Coverage   76.71%   76.73%   +0.02%     
==========================================
  Files          23       23              
  Lines       11155    11227      +72     
==========================================
+ Hits         8557     8615      +58     
- Misses       2598     2612      +14     

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