feat(grid): add sample for Header Grouping spanning accros multiple cols

[ghiscoding]

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[ghiscoding]

@jmzagorski
You can review if you want and have time, I'll probably merge on Monday and look at a new version during the week before I leave in vacations.

Some things worth noting about this PR, Grouping Header doesn't work so well with a window resize. Probably some CSS to fix up somewhere. Also, the pre-header and header-row have to be re-created every time the grid changes (like sorting, resizing column width, ...) which is dealt properly in FilterService with grid.onHeaderRowCellRendered which calls addFilterTemplateToHeaderRow but for the pre-header I found out there was a lot more to do on that side of it and it isn't yet correct with grid resize.

I managed to do the following over the weekend

• add support for multi-columns number indicator & tristate (in master)
• add Column Span (colspan) demo (in master)
• add Header Grouping spanning across multiple cols (this PR)

I'd like to get this done too, which I'm about to start working on

• Copy & Paste like Excel, see SlickGrid demo

If you have something upcoming, let me know. The reference list #31 is getting smaller and smaller :)