Link enabled state should be mutable

[mindofmatthew]

The typical implementation for Ableton Link support is for it to be a feature that can be dynamically enabled and disabled. I think we should support this behavior via a function like setLinkEnabled True or something that can be called with an existing stream/clock.

In addition, I personally think that the default setting should be for Link to be disabled so that people aren't connecting to Link sessions without realizing it. I recall reading some bit of Link documentation that suggested this was a mandatory default, but I can't find it any more, so their guidance may have changed or I may not be remembering correctly.

[polymorphicengine]

added this ability in the current PR: 3fedf10

also set the default to disabled for now, as i also think it should be the default, but i guess we can still discuss it a bit