implement From<String> for Element

[JL710]

This PR adds a From<String> implemenation for Element.

This works:

widget::button("")

This did not work:

widget::button(String::new())

You needed this:

widget::button(widget::text(String::new()))

This PR makes it a lot less redundant if you want have format! generated text in a button.

[derezzedex]

this PR #2338 might interest you, it's a text macro analogous to the format macro.

[JL710]

Awesome macro. I will replace abount 100 occourences in my code with it.

[hecrj]

It's unclear when this From implementation may be useful; so I am closing this in favor of the new text! macro introduced in #2338.