Implement Focusable

[Haprog]

Make DateTimePicker implement Focusable<DateTimePicker> and make sure focusing works. It should probably focus the date picker. This did not work as is when implementing #1 so need to investigate what other changes are needed to make it work. Test if focusing works on client side (may require changes to vaadin-date-time-picker).

Related comment: https://github.com/vaadin/vaadin-date-time-picker-flow/pull/4/files/13cbc490bed85d7a45e42c957e6f1811b0be72e1#r333962898

[Haprog]

This might be easy to implement now that vaadin/vaadin-date-time-picker#25 is done.

Hopefully now the only thing that needs to be done for this is to add:

import com.vaadin.flow.component.Focusable;

And Focusable<DateTimePicker> into the implements section of DateTimePicker class. Then just need to test that calling dateTimePicker.focus() (e.g. from button click) actually focuses the date picker.