Implement a sound source tracking mechanism

[irydacea]

Currently, there is no mechanism provided by the Wesnoth engine to list existing [sound_source]s according to any given set of criteria. This makes it awkward at this time to disable or enable sound sources temporarily in the middle of a scenario.

A possible solution that doesn't require modifying the engine is wrapping the implementation of [sound_source] in Lua so that each time a sound source is configured, it is added to a table (WML variable, say, $__lua_AtS_sound_sources) that can be queried later and potentially filtered by some criteria. [remove_sound_source] could use additional filtering like that.