feat: add optional Top-Header for Drag 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.
• also created a new example-draggable-header-grouping.html