Add change event

[tomivirkki]

Fixes #207

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This change is 

[limonte]

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

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Comments from Reviewable

[manolo]

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

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src/vaadin-date-picker-mixin.html, line 795 at r1 (raw file):

    /**
    * Fired when the user commits a value change.

nit jsdoc lines needs a extra space except first line

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Comments from Reviewable

[manolo]

apart from formatting nit. Not blocker though

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

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Comments from Reviewable

[tomivirkki]

src/vaadin-date-picker-mixin.html, line 795 at r1 (raw file):

Previously, manolo (Manuel Carrasco Moñino) wrote…

nit jsdoc lines needs a extra space except first line

Done.

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Comments from Reviewable

[manolo]

Review status: 3 of 4 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[manolo]

Review status: 3 of 4 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[manolo]

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

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Comments from Reviewable