Set up volume control code in webkit plugin for platform-specific implementations

Renamed LinuxVolumeCatcher class to VolumeCatcher.  Everything in the header except the class name was already platform-agnostic, so I just renamed it to volume_catcher.h.

Moved the stub implementation from the end of linux_volume_catcher.cpp (previously only used if LL_PULSEAUDIO_ENABLED was not defined) to dummy_volume_catcher.cpp, and made CMakeList.txt file use the dummy impl on Linux if PULSEAUDIO is not defined.

Removed the ifdefs on the use of VolumeCatcher in MediaPluginWebkit, so it gets called on all platforms.

Added a setPan() function to VolumeCatcher (currently unimplemented, but we'd like to have this capability in the future).