Use `Magento\Framework\Filesystem`, to allow override by RemoteStorage

[lphilippo]

While working on a plugin to integrate this module with Magento's RemoteStorage, we'd like to propose a few changes to increase the flexibility for such inheritance.

By using Filesystem $filesystem, a module can link this constructor parameter by their standard way:

    <type name="Yireo\NextGenImages\Image\File">
        <arguments>
            <argument name="filesystem" xsi:type="object">fullRemoteFilesystem</argument>
        </arguments>
    </type>

For the private properties, only the $fileDriver itself is most likely to be needed, hence the change from private to protected. With these changes the integration works correctly, although we're still in the process of testing the different features (most importantly the direct conversion on upload).

[jissereitsma]

I didn't really put much thoughts to it. Adding it and releasing it right away!

[jissereitsma]

Included in 0.2.12 :)