Allow access to disclosure state in code

[henribru]

Currently you can access the disclosure state in the template using a slot prop, but it would be useful if you could also get access to it in code, either by having it work through v-model like Switch does or keeping the state internal but emitting an event when the disclosure state changes (the latter is a bit less flexible since v-model would also allow setting the initial state). My specific use case is to lazily fetch data once the disclosure is opened, though I'm sure there are other cases where this could be useful as well.

Though I haven't had a specific need for it, this might be useful for Popover as well.