Selecting automation items leaves items in other envelopes selected, count is reported incorrectly when deleting

[jcsteh]

Spun off #394 (comment).

I always assumed automation item selection was only relevant for the currently selected envelope. This is true for points; e.g. if you select some points on a volume envelope and then select some points on a pan envelope, deleting points only deletes the points on the pan envelope.

It turns out that when you select an automation item, just like media items, it stays selected even if you switch envelopes. That means if you move to an automation item on a volume envelope and then move to an automation item on a pan envelope, pressing delete will actually delete both of them, even though we only report that one of them was deleted. Big ouch.

I'm going to need to fix this at a deeper level before I tackle splitting, etc. because otherwise, we're going to incorrectly select multiple items and incorrectly report how many are being manipulated, furthering the problem we already have for deletion.