ecmascript: Add support for item hooks

Fixes #2415
ecmascript: Add support for receiving 'propref' events on prop subscriptions
ecmascript/prop: Add support for creating and sending 'openurl' prop events
ecmascript: Add some missing support functions

Showtime.selectView()
Showtime.fs.copyfile()
Showtime.fs.basename()
ecmascript/settings: Give raw access to properties of created setting
ecmascript/setting: Make it possible to assign current value of a setting