DatePickerElement: clear() not working, when element uses "autoselect"

[stefanuebe]

The clear implementation of the date picker tries to select everything and then clear the input using different keys. The problem is, that it does not check, if the element might have already the input selected. In that case the key inputs will not select everything, but simply move the cursor to the beginning.

To ensure that everything is always selected it should send the "END" key first, before using "SHIFT+HOME"

[yuriy-fix]

Might affect other component's clear API.