Add 'clear' button on developer-tools/template

[Mariusthvdb]

as title, which speaks for it self.
The current UI is a pain when using non-mouse interface to the page. Especially mobile is practically unusable because of this.

imagine a older template pasted inside the dev tools widow, and wanting to test another, requiring to clear the page first. we now have to backspace the whole field.....

a 'clear' button would make life so much easier, and there is space for that on all conceivable interfaces/devices.