Support column groupings in DataGrid widget

[Flynsarmy]

Large datagrids can look very cluttered and disorganised:

I propose adding a tab option for columns which would result in an extra header row and cell borders like so:

Example markup:

my_field:
    label: Option Matrix
    type: datatable
    searching: true
    columns:
        foo:
            title: Foo
            tab: Options
        bar:
            title: Bar
            tab: Options
        is_disabled:
            title: Disabled
            type: checkbox
            tab: Product
        sku:
            title: SKU
            tab: Product
        images:
            title: Images
            tab: Product
        price:
            title: Price
            tab: Pricing
        cost:
            title: Cost
            tab: Pricing
        is_on_sale:
            title: On Sale
            type: checkbox
            tab: Pricing
        sale_price:
            title: Sale Price or Discount
            tab: Pricing

Some thoughts on this proposal:

• group would perhaps be a better option name, however that word could potentially be used in the future for collapsing/combining or some other form of grouping of columns. tab is consistent with how FormField definitions work.
• If one or more columns have a tab and others do not, the extra header row will still appear and the unnamed columns will just be placed into their own blank groups like so (Disabled and Images columns have no tab):
  
• This proposal is backwards compatible, as no existing datatables have columns with a tab option, and the extra header row will not appear unless at least one column has a tab.

I'm happy to work on this as a PR - just gauging the likelihood of something like this being accepted first.

[LukeTowers]

Looks nice! @bennothommo any thoughts?

[bennothommo]

@Flynsarmy I love it. My only feedback would be I think group is the better term (and has some precedence as the Inspector widget uses group as well to group fields together). I'd almost say tab would be better for your potential collapsing/combining functionality. But feel free to proceed on the PR based on what you've described above.