Add new interface for user selection of managed files

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

We need a view where users can find the media they need, select that media, and "submit" their selection. When submitted, the form returns the fid of the selected file to the caller.

This type of UI will be a fundamental aspect of any media library solution we create.

From one perspective, views bulk ops already provides much of the needed UI for this issue.

• Integrated with views
• Single-item selection with radios
• Multiple-item selection with checkboxes

But we don't exactly need to execute an operation on the selected items. It might take a bit of tinkering to get VBO to work perfectly for this need, but likely not impossible.

Thoughts / feedback?