Replace event types "custom" and "run_once" by "on_trigger"

The former "run_once" events have an additional "repeat=false" event
component.
Replace "stash" event by "on_trigger" event, add "stash=true" again
spawn maps events type changed to "on_load" instead of "custom"
Replace "run_always" events by "on_trigger" events
Fix soundfx in brothers lair ice hall

Renamed event component "sound" to "soundfx"
Merge pull request #310 from stefanbeller/events

Reorganize event types.
This implements the engine changes as of clintbellanger/flare-engine#633