Simplify the list of available backends and names

Define a constant string for the backend name, instead of
a function which returns it.  The name can be a simple C string
because it is only used internally.  Having done this, the name
can be stored in the g_mixerFactories list instead of the
function, making the lookup in backendNameFor() much easier and
eliminating having to count through the list in numBackends().

The backend virtual function getDriverName() can also just return
this static string, so avoiding repetitive repetition of the name.