feat: add clear-button-visible API from <vaadin-text-field>

[platosha]

Fixes #645

BREAKING CHANGE: The clear button is now hidden by default. To make it
visible, set the clear-button-visible attribute or use
clearButtonVisible JS API property.

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This change is 

[Haprog]

Should probably also bump vaadin-text-field dependency version in bower.json from ^2.1.2 to ^2.3.0 since v2.3.0 introduces the clear-button-visible API.

[platosha]

@Haprog done

[Haprog]

@platosha Looks like in this version when you click on the clear button, it always opens the date picker, regardless of if the date picker input was focused or not and if the overlay was open or not. This is different from current behaviour in released version.

If you test it here: https://cdn.vaadin.com/vaadin-date-picker/3.3.2/demo/#date-picker-basic-demos clicking on the clear button doesn't open or close the date-picker, it just clears the value and focuses the text field. So if you click the clear button when the overlay is open, it stays open. And if you click the clear button when the overlay is closed, it stays closed.

We should probably add tests for this so that the behaviour doesn't unintentionally change. So we should test what happens when the clear button is clicked in at least these cases:

Clear button is clicked when:

1. date picker is not focused, overlay is closed
2. date picker input is focused, overlay is closed (e.g. right after selecting a date from the overlay which closes the overlay but leaves the text input focused)
3. date picker overlay is open