Create new form that wraps the "file/add" and "Media Library Selector" forms for re-use

This relates to File entity and re-usable managed files system #1448

In most cases when dealing with a mechanism for re-usable files, the user will need the option to select an existing managed file, or upload a new file. The alternative is that the user is only given the option to select an existing managed file and must browse to another location to upload a new file. Which seems less that ideal.

This task is to be completed after #2633 (a UI for uploading new managed files) and #2718 (a Media Library Selector view).

Task goal is to create a simple wrapper for the above two forms that is easily reusable. Specifically for #1307 (new file field widget) and #2719 (modify CKEditor image dialog form)

As we add new features to the file entity system (such as sideloading files from external URLs), those options should be included in this wrapper form.