migrate contents file on first properties access

[kevinushey]

This should resolve #1623. I believe this is a candidate for the patch release. Copying motivation from that thread:

---

I was able to reproduce this issue in the following way.

1. Create an RStudio project,
2. Open that project with RStudio v1.0,
3. Open a few files in that project,
4. Shut down the IDE,
5. Open the project with RStudio v1.1,
6. Try editing one of the open files,
7. Close and re-open RStudio v1.1,
8. Notice that the open document is empty.

I think this is because we don't fully migrate to the new format on the first transfer of files. On the first open of RStudio v1.1, the sources folder looks like this:

$ tree
.
├── per
│   ├── t
│   └── u
├── prop
│   ├── 2646FB5C
│   ├── 28A9DEA3
│   ├── DC6A99FB
│   └── INDEX
└── s-F955001D
    ├── 244D0217
    ├── 88798333
    ├── FAE2342D
    └── lock_file

After a close + re-opening of the IDE, it looks like this:

$ tree
.
├── per
│   ├── t
│   └── u
├── prop
│   ├── 2646FB5C
│   ├── 28A9DEA3
│   ├── DC6A99FB
│   └── INDEX
└── s-1794F329
    ├── 244D0217
    ├── 244D0217-contents
    ├── 88798333
    ├── 88798333-contents
    ├── FAE2342D
    ├── FAE2342D-contents
    └── lock_file

Because those -contents files aren't populated on the first run, we end up with property files written out with no contents after an autosave. To mitigate this, we should extract the contents out of each properties file during migration and write the associated -contents file.

[jmcphers]

Nit: spelling (properties)

[kevinushey]

Doh; thanks! Fixed with an amended commit.