No individual shadow roots for each separate cell, better content selectors

[tomivirkki]

These fixes mainly target grid with native shadow DOM on:

• Each cell doesn't need to have individual shadow roots (just the content selector)
• Class names aren't the right choice for content selectors in this case, using attributes
• Light dom items shouldn't accumulate on dom changes (for example when number of columns change)
• vaadin-grid-table-cell implementation is now more in line with other vaadin-grid-table- components

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This change is

[Saulis]

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Reviewed 2 of 2 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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vaadin-grid-table-cell.html, line 126 at r1 (raw file):

          // not helping here for some reason.
          this.async(function() {
            if (this.target.domHost) {

Comment: looks like this logic doesn't work perfectly when grid is nested inside a dom-bind template, domHost is then null. However, vaadin-grid seems to be assigned with a x-scope class. I'll file a separate issue about that.

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