Add focus and blur methods, focused property

[platosha]

Fixes #349

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This change is 

[limonte]

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Comments from Reviewable

[manolo]

I miss the tabindex property that should have any form control. But after experimenting with it, I don't found a 100% working solution. So let's leave this for a separated PR because other elements also have the same problem.

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Comments from Reviewable

[tomivirkki]

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vaadin-combo-box.html, line 407 at r1 (raw file):

    },

    _onInputFocus: function() {

This isn't used

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test/vaadin-combo-box-properties.html, line 209 at r1 (raw file):

        it('should focus the input with focus method', function() {
          var inputFocusCalled = false;

Unneeded variable. sinon.stub returns an object whose called property can be asserted. These could also be moved to beforeEach since they're used in 2 tests

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Comments from Reviewable

[Saulis]

I too think this should work with tabbing. Here's a prototype proposal that enables tabbing both in Shadow and Shady: https://jsfiddle.net/Saulis/8mj7xzfa/

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Review status: all files reviewed at latest revision, 2 unresolved discussions, some commit checks failed.

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Comments from Reviewable

[manolo]

Smart solution @Saulis, I added it to my tabindex conclusions here:
https://github.com/vaadin/vaadin-button/blob/proto/native-button/tabindex-research.md

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Review status: all files reviewed at latest revision, 2 unresolved discussions, some commit checks failed.

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Comments from Reviewable

[platosha]

@Saulis is it ok to add this in a separate PR?

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Reviewed 2 of 2 files at r2.
Review status: all files reviewed at latest revision, 2 unresolved discussions.

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vaadin-combo-box.html, line 407 at r1 (raw file):

Previously, tomivirkki (Tomi Virkki) wrote…

This isn't used

Done.

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test/vaadin-combo-box-properties.html, line 209 at r1 (raw file):

Previously, tomivirkki (Tomi Virkki) wrote…

Unneeded variable. sinon.stub returns an object whose called property can be asserted. These could also be moved to beforeEach since they're used in 2 tests

Done.

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Comments from Reviewable

[Saulis]

Yes I'm OK with that, I can add a separate ticket for it.

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Review status: all files reviewed at latest revision, 2 unresolved discussions, some commit checks failed.

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Comments from Reviewable

[tomivirkki]

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Reviewed 2 of 2 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable