Fixed EZP-23080: added inputUri property to Image\Value

> Status: work in progress
> Issue: http://jira.ez.no/browse/EZP-23080

Similar to #881, but for images.

`Image\Value` used the `id` property for both input and output. It made it hard to distinguish wether the provided value referred to a new image or to an already persisted one.

This change introduces `inputUri`, dedicated to provided an input file. `id` can still be used, but an `E_USER_DEPRECATED` error will be thrown (this is unit tested, and the code (see 78708d1) might be worth using for testing deprecation !).

`path` is now mapped to `inputUri` when used for input.
Well, trigger_error with E_USER_DEPRECATED sounds pretty standard to me...

And it is the only way to test it that I've found. But yes, the error handler should be changed back, not sure why I skipped it :-)