Fixes missing ".wav" or ".ogg" from export

When exporting OGG or WAV and a file extension is not provided, the export settings dialog never gets the proper file extension. Issue #764.  In addition, this bug causes the settings dialog to forget whether ".ogg" or ".wav" was selected in the previous dialog.

This fix parses the `(*.wav)` from the QFileDialog and appends it as a file extension suffix to the exported file name.

This change was tested on Ubuntu 12.04 and does NOT impact the Export Tracks option.

![image](https://cloud.githubusercontent.com/assets/6345473/3108541/72b7d098-e691-11e3-9216-0891bae4d8e0.png)

![image](https://cloud.githubusercontent.com/assets/6345473/3108545/7ed1762c-e691-11e3-83c9-a4ad4f57e8a6.png)