Remove OpenAL dependency from sound manager public API

[historic_bruno]

Currently, any module that wants to use the sound manager has to add OpenAL as a dependency in the build system, which is undesirable. Instead, only the sound manager's module should have that dependency and it should abstract the OpenAL implementation details so other code need not worry about it.

ok, this should be fixed as of r13455. Now code external to the soundmanager should include ISoundManaer.h, which won't have any dependencies.

[stwf]

ok, this should be fixed as of ​r13455. Now code external to the soundmanager should include ISoundManager.h, which won't have any dependencies.