Splitted Image\Value into Base,Value and Input

> See [EZP-23080](https://jira.ez.no/browse/EZP-23080)
> Status: work in progress

### `Image\Value` changes
- `Image\BaseValue` is abstract, and holds user-defined attributes (`alternativeText` & `fileName`)
- `Value` is built only by the API, and contains `id`, `imageId`, `fileSize` and `uri`
- `InputValue` is used for inputing a new image, and adds an `imageUri` property. It checks, when given an `inputUri`, than it is a valid local image file.
Nitpick: 
Should there be a check for the existence of these array keys? (or could this throw a PHP notice otherwise?)