Make defining sound_event json files optional

[bchantech]

Currently, sounds are specified via sound_event, however, most if not all of the time it only contains path containing the sound file of the same name.

When the game plays a sound event that is not defined, it should try to play the sound file of the same name using the default parameters.

Example:

/sound_events/sound1.json is specified in UI

If sound_events/sound1.json is not present, it should create a new sound_event with the following info:

{
	"path": "sounds/sound1.ogg"
}

If the game cannot find the sound, then path will not be included.
If the game can find the sound, and detects it is stereo, flat will be set to true.

[jakubg1]

I will consider this during the sound event overhaul (#95). This might make resource handling more complicated though (passing sound files directly instead of sound events means that all class configs will have to check whether it's a sound or an event; on the other hand, specifying deliberately nonexistent sound event files might be too unintuitive).

ResourceManagers may be able to create sound events on the fly when a path to the sound file is passed in a :getSoundEvent() call, though.

[bchantech]

I'm closing this since with ff71f34 sounds can be placed in any directory so we can't assume that they will be present in the /sounds folder.

[jakubg1]

Reopening since providing a path to a sound file whenever a Sound Event is expected should generate an implicit Sound Event to avoid creating unnecessary boilerplate Sound Event files of which the only purpose is to exist - as a QoL for modders so that they need to change/create one file less.
This part has nothing to do with the resource placement, as you need to provide a full path to any resource anyways.

[jakubg1]

Also, relates to #123 and is a part of a bigger plan to iron out all inconsistencies in the way expressions are defined, resources are referred (#103), etc. that will be carried out throughout the rest of the Beta 4.x cycle.