Treat PulseAudio capture devices/streams as capture instead of playback

So that they show up as such in the GUI, with the "capture/off" icon
instead of the "speaker/mute" one, and labels and tool tips as
appropriate.

Use lambdas and some helper functions to simplify Mixer_PULSE::readVolumeFromHW()
and Mixer_PULSE::writeVolumeToHW().