add example of a mat-table with re-orderable columns

[jeanbza]

This adds an example of how to re-order a table's columns with CDK drag drop.

This example can be seen at https://stackblitz.com/edit/mat-table-col-reorder. (I created that - it is almost 1:1) (It does not work since stackblitz doesn't have the fix for #15948)

This has come up numerous times in issues and requests:

• https://stackoverflow.com/questions/53377450/reorder-mat-table-rows-with-angular-materials-drag-and-drop
• Drag and Drop with MatTable #13770
• https://embed.plnkr.co/DevUDv/
• https://medium.com/vendasta/wrapping-angular-material-table-styling-it-once-drag-drop-sorting-b1765c995b40
  (etc)

Note: It looks like CdkDragDrop.previousIndex is never being updated. It always reports the original index - rather than the previous - of the thing being dragged. So, in this PR I've had to add a little mapping from CdkDragDrop.previousIndex -> name from column list -> actual previousIndex.

Note: This requires a very recent version of angular that has the fix for #15948 and #18482 (as of writing, that is likely to be HEAD - hopefully fixes are released soon).

[jeanbza]

Refactored to not have the hack now that the fix is in. Also cleaned lint errors. PTAL.

[crisbeto]

LGTM

[Falven]

@jadekler could you go into more detail on the version of angular required?

[jeanbza]

@Falven I've updated my PR description, but basically it's HEAD currently. Hopefully those bug fixes are released soon.

[jelbourn]

@jadekler FYI to show up on the docs site, we also need to add this entry to https://github.com/angular/material.angular.io/blob/master/src/app/shared/documentation-items/documentation-items.ts#L410

[jeanbza]

@jadekler FYI to show up on the docs site, we also need to add this entry to https://github.com/angular/material.angular.io/blob/master/src/app/shared/documentation-items/documentation-items.ts#L410

Done angular/material.angular.io#727

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