Input component should support moving the cursor via tapping

<a href="https://github.com/Hixie"><img src="https://avatars.githubusercontent.com/u/551196?v=3" align="left" width="96" height="96" hspace="10"></img></a> **Issue by [Hixie](https://github.com/Hixie)**
_Monday Jul 20, 2015 at 20:33 GMT_
_Originally opened as https://github.com/flutter/engine/issues/152_

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_From @Hixie on April 17, 2015 21:31_

_Copied from original issue: domokit/mojo#100_
