Enhancement: Streamline OcTextInput

We have updated the OcTextInput component to be in line with other form components in the design system.

- Fixed the clear button being visible even when element is disabled
- Made clear button emit null instead of empty string
- Made input/change event handling a bit more consistent
- Added defaultValue prop that is for now passed as placeholder to the input field

https://github.com/owncloud/owncloud-design-system/pull/1636
