Add preamp setting with a default of 6 dB

This matches the previous behavior that renders audio at a level comparable to other playback apps on iOS and tvOS. It effectively reverts f68376ba.

When resetting an equalizer profile, the newly configured default preamp level will also be respected.

Hereby, an option is introduced to further increase the preamp level but also to lessen it to 0 dB and below to retain audio quality without compression at the expense that other apps on the same device might be louder leading to an incoherent playback experience.

libVLC allows a preamp level of up to 20 dB. This patch limits the level to 12 dB to prevent users from breaking their device speakers and more notably cheap Bluetooth device incapable of handling the amount of compression.

This patch fixes #374 (closed) and #1423 (closed)