DeviceWidget and StreamWidget had some duplicate code to initialize
ChannelWidgets. This patch moves some of the duplicated initialization
code into ChannelWidgets to reduce repetition and to improve
encapsulation.
This allows us to set volumes up to ~153% aka +11dB.
Also show the current dB value in the UI - as pavucontrol is a bit more
developer-friendly than other volume UIs displaying this by default makes
sense.