Fix #4246: RTL text plugin caused leak on Style#_remove()

[ChrisLoer]

The unit test verifies that the "deregister" call happens, but because actually exercising the plugin-loading code depends on the network, I fell back to manual testing.

I tested by creating two maps, verifying there were two callbacks registered, removing one map and seeing that the second callback was removed, then calling setRTLTextPlugin to verify the remaining callback was successfully called and then discarded.

@jfirebaugh @mourner

[lucaswoj]

This is starting to feel like an implementation of Evented. Would it be possible to use Evented to manage plugin available callbacks? That code is well tested and believed to be memory-leak free.

[ChrisLoer]

@lucaswoj I switched to usingEvented, but had to modify the implementation Evented.once() so that off() works on one-time listeners as well as listeners added with on().

I also modified Evented.listens() so that it wouldn't return true when listeners had been added and then removed. The rest of this PR doesn't depend on that change, but at least based on my reading of the function documentation, it's the expected behavior.

[lucaswoj]

Looks good! Thanks for the bonus bugfix and extensive test cases.