Improved PipeWire and PulseAudio support

• Use libpulse (when available) for displaying the list of selectable audio input devices by default, instead of alsalib for listing audio inputs and pactl for switching to a monitor mode device, so that the list of devices displays correct values on PipeWire/PulseAudio-based systems
• Make the dependencies over libpulse and DBus MPRIS fully optional, through Cargo features
• Disable MPRIS signalling in default builds, as it may interfer inopportunely with media keys control from other media players (as submitted in issue #89)
• Fix build issues on non-Linux platforms (thanks to pull request #144 from @dotX12)