Support the clear() method on TestBenchElement.

[jorgheymans]

Also reported here https://vaadin.com/forum/t/clear-value-for-vaadin-text-field/160779/2

It would be nice to be able to do $(TestBenchElement.class).id("abc").clear() as the api allows to. Currently we're getting org.openqa.selenium.InvalidElementStateException: invalid element state for this.

The workaround works though: sendKeys(Keys.chord(Keys.CONTROL, "a"), Keys.DELETE);