Writing support for Default Filename in Add New File dialog.

The current code used the DefaultFilename tag which was unused in the XMLs, while the XMLs had an
attribute for the FileTemplate tag, which is unused. I've decided to use the DefaultFilename version,
due to not messing around with code or how FileTemplate works. This change is useful for creating
quick test projects. The functional change found [here](https://github.com/mono/monodevelop/pull/272/files#L42R380) makes it so it switches between default names
but keeps you from losing written file names.