[winecfg] speed up audio driver loading

[Damjan Jovanovic]

These changes reduce the time needed to load the Audio tab of winecfg
from 12 seconds down to 5.

Changelog:
* Load audio drivers in multiple threads, so the length of time the
GUI is blocked for is the length of the longest driver load time,
rather than the sum of them all.
* Keep audio drivers open until they need to be reloaded, because it
can take a very long time to load them multiple times.

Damjan Jovanovic
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