Add mix-ins to allow external styling of data-table-cell elements

[depeele]

This pull request adds mix-ins to allow external styling of data-table-cell elements, both normal cells and those within a header.

• normal cells : --iron-data-table-cell
• header cells : --iron-data-table-header-cell

The change is primarily to allow resetting the size of header-cell height enabling the use of elements within the header that exceed the hard-coded height.

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This change is

[Saulis]

Not sure these are needed, user should be able to style the elements just withdata-table-cell and data-table-cell[header] CSS selectors?

[Storyyeller]

I think this is necessary when using iron-data-table with shadow dom. Ordinary CSS selectors won't pierce the shadow dom, but CSS variables can.

[Saulis]

@Storyyeller <data-table-cell>, <data-table-row>, <data-table-checkbox> elements are being exposed in the light DOM of <iron-data-table> by design, so no shadow DOM piercing is required.

See example here: http://jsbin.com/fetibiy/edit?html,output

[Storyyeller]

Oh, I see what you mean. I think my problem was that we were using ::content in our selectors, but the cells are not actually inside a content tag.

[Storyyeller]

I got it working now. Thanks for the help and sorry for bothering you.

[Saulis]

No problem at all, glad you got it working! 👍🏻