Support desc attribute on index element

[jtackaberry]

The reapack element provides an optional desc attribute which is used as the display name in lieu of name if specified.

Similarly, it would be nice if the index element supported this as well. My use case is for REAPER Toolkit, which is a library. I want the on-disk path to be .../Scripts/rtk/... to provide a convenient and well-known path for importing by other scripts, but to display "REAPER Toolkit" in the Manage Repositories dialog.

I'd like to be able to define this:

<index version="1" name="rtk" desc="REAPER Toolkit (rtk)">
...
</index>