add import to contractor option depening on file type

Let's say I download a picture from Google. It's saved in Downloads.
Now I navigate to Downloads and to get the Picture into Pantheon-Photos I've to for example copy the image into the Pictures directory.
Faster would be a contractor option "Import to Pantheon-Photos". So I can select all my Pictures and import them into Photos. Obviously in the background they would be copied into Pictures.
I say depending on file type because this scenario also works with music files by importing them into Noise.

Launchpad Details: [#LP1480386](https://bugs.launchpad.net/bugs/1480386) swizzle - 2015-07-31 16:17:24 +0000

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No files, tests, or entry points are named. Start by tracing how Pantheon Photos handles imports and how Contractor actions are registered, then define the file-type behavior and verify that selecting files offers the appropriate import action and copies them into the Photos or Noise library.

Written by the indexing model from the issue text.