Validation rounds

[yuriy-fix]

vaadin-date-picker runs several validation rounds on setting the value and that leads to several small issues (i.e. invalid state changes firstly to true and then to false when setting the value for the first time). change event is dispatched before last validation round is proceeded (so the invalid state could be incorrect at that point of time).

We should rethink the logic of the validation and get rid out of several validation rounds.