Input "enabled/disabled" API and documentation consistency updates

Updates wrt  https://github.com/photonstorm/phaser/issues/1191 - backwards compatible, no known issues.

The `disabled` property of various input classes has been inverted to `enabled` and direct/internal usage of `disabled` has been transformed to `!enabled`. This mirrors the positive-case usage found in almost all API: visible vs. hidden, exists vs notExists, etc.

A `disabled` get/set property has been added to wrap `enabled`, and marked as `@deprecated`. This ensures current API compatibility.
+1 docs are much more clear and concise.