Add texture manager ADD_KEY and REMOVE_KEY events

[samme]

The existing ADD and REMOVE events are a little inconvenient if you're interested in one texture, as you have to check the key and then (probably) remove the listener when finished.

Dynamic event names would let you use once() for this.

this.textures.once('addtexture-dude', (texture) => {/* … */});

this.textures.once('removetexture-dude', (texture) => {/* … */});