Implement module-always-source

Implement module-always-source

This is basically a copy of module-always-sink but doing the same for
sources. Whenever no source is available, a module-null-source is loaded
and whenever a new source is available again, module-null-source is
unloaded.

By this, anything using a source will automatically be switched to the
null source when the actual source disappears, and back to the actual
source if it appears again.