Remove default horizontal padding for Input. - #6437

LGTM.

Note that a side-effect here will be that tapping the edge of an input that has padding won't work any more, since the gesture detector is moving from outside the padding to inside the padding.

An alternative solution would be to just make the padding on the input configurable.

Yeah, I considered adding a padding argument to Input, but it seemed unnecessary and inconsistent (would all material controls have controllable padding?). Also, seeing it in practice, I don't expect a tap on the empty padding space to focus the Input control, so that doesn't seem like an issue to me.