StructBondModule tweaks/fixes

[disberd]

This PR contains some small fixes/changes to the structbond module aimed at simplifying structure and closing some issues:

• The CSS code is now stored in separate CSS files for better readability/maintainability
• The class name of the bond table which had a typo was fixed, closing Style from @BondsList can break BondTable layout #19
• The popout container now gets hidden with a delay of 300ms after unhover, and it will stay displayed if the mouse either re-enters the popout header, or enters the popout contents area, making it easier to modify some popout bonds on the fly without having to pop out the container. This closes Make the Popout not hide automatically when hovering over its contents #44
• Any julia object which can be represented as HTML is now a supported description for @NTBond. This includes markdown with math inside, as per screenshot below, effectively closing Cannot do LaTeX or markdown in description of @NTBond #13
• Added the possibility of simplifying application of PlutoUI.Experimental.transformed_value to the fields of an @NTBond using the @tv decorator (see the example notebook for details). Closing Add support for transformed_value function also for StructBond fields #43

Markdown math in @NTBond description

transformed_value for fields bonds

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.98%. Comparing base (e786e41) to head (7b54176).
⚠️ Report is 17 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   98.94%   98.98%   +0.04%     
==========================================
  Files          16       16              
  Lines         758      791      +33     
==========================================
+ Hits          750      783      +33     
  Misses          8        8              

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