Implement focus-trap

[limonte]

In #13 we decided to move focus-trap functionality (which will be required by vaadin-dialog) to vaadin-overlay.

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This change is 

[limonte]

Tests are finally working in all browsers, ready for review 🚀

[tomivirkki]

Seems to be working well 👍

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Review status: 0 of 5 files reviewed at latest revision, 2 unresolved discussions.

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vaadin-overlay.html, line 302 at r2 (raw file):

      _getFocusableElements() {
        return Array.prototype.slice.call(

Prefer Array.from(...querySelectorAll...) with array-like objects in es6

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vaadin-overlay.html, line 304 at r2 (raw file):

        return Array.prototype.slice.call(
          this.$.content.querySelectorAll(
            'button, input:not([type=hidden]), textarea, select, a, *[tabindex]:not([tabindex="-1"])'

The set is different from https://github.com/vaadin/vaadin-grid/blob/master/vaadin-grid-cell-click-behavior.html#L63 Probably both sets have some flaws

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Comments from Reviewable

[limonte]

Review status: 0 of 5 files reviewed at latest revision, 2 unresolved discussions.

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vaadin-overlay.html, line 302 at r2 (raw file):

Previously, tomivirkki (Tomi Virkki) wrote…

Prefer Array.from(...querySelectorAll...) with array-like objects in es6

Done, thx!

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vaadin-overlay.html, line 304 at r2 (raw file):

Previously, tomivirkki (Tomi Virkki) wrote…

The set is different from https://github.com/vaadin/vaadin-grid/blob/master/vaadin-grid-cell-click-behavior.html#L63 Probably both sets have some flaws

Good point, it's good to be aligned in such cases. I copied the set from Grid with a small additional check for tabindex="-1"

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Comments from Reviewable

[tomivirkki]

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Reviewed 2 of 5 files at r2, 3 of 3 files at r3.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[yuriy-fix]

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Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable