Misc improvements based on user feedback

[AlmasB]

From #1150:

• [ ] FXGL.getAudioPlayer() could have removeAllMusic and/or Sound, it could also have get method returning sounds/music that "are there" or what is currently "playing"
• [x] missing method removeAllEventHandlers() on FXGL.getEventBus() for restarting games
• [ ] to get list of existing saved games, it is required to load the content of saved files. It should be possible to save metadata separately.
• [x] ~loading a generic asset from the asset loader~ there is AssetLoader.getURL(String) that returns a URL to a generic asset
• [ ] not possible to change cutscene actors' image size

[AlmasB]

@jmaler removeAllEventHandlers() is now present in the latest build.

[jmaler]

Great, thanks!