feat: add optional Top-Header for Draggable Grouping & Header Grouping

[ghiscoding]

• prior to this PR, we could not use Draggable Grouping & Header Grouping because both features were using the pre-header and it would have conflict, now to avoid this I am adding an extra Top-Header that will show over all header and pre-header.
• NOTE: Draggable Grouping can now be displayed in either the new Top-Header OR the Pre-Header. However, the users who do want to use both features will need to explicitely enable both top/pre headers.
• I was getting a little out of ideas to get a name (since "pre" is already taken), so I decided to go with Top-Header which is completely on the top and even before the pre-header.
• Another note is that this new Top-Header is not following the frozen columns (like header & pre-header do) and is rather always the same width as the grid. I purposely did not want this top-header to follow the grid frozen columns because I wanted a simpler header and also because I expect this to be mostly (and only) be used by the Draggable Grouping.

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 22.37%. Comparing base (c8122e7) to head (377a484).

Files	Patch %	Lines
...slickgrid/src/custom-elements/aurelia-slickgrid.ts	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1219      +/-   ##
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- Coverage   22.42%   22.37%   -0.04%     
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  Files           8        8              
  Lines         901      903       +2     
  Branches      330      310      -20     
==========================================
  Hits          202      202              
- Misses        699      701       +2     

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