MultiCursor: make events connected during __init__ accessible (for later removal)

In a custom application where I repeatedly clear figures and reinitialize a `MultiCursor` object I need to inherit and override its `__init__` method (which I now have to maintain with `if`/`else`s depending on installed matplotlib version because of changes in upstream, like 349ccecb64) because I need to disconnect the events again.
This would not have been necessary on my end had `MultiCursor` been aware of the IDs of its connected events. So, I think this might be useful for other people and it would make it possible for me to reuse `__init__` for future matplotlib versions.

(Or maybe I just don't see the easy solution to disconnect these events again without saving their IDs?)