apiset: make ObjectSet.AddSetForType merge instead of replace

[landism]

Problem

ObjectSet.AddSetForType will blow away any existing objects of the given type, but it's also much easier to read/write than looping over objects.

In the Tiltfile reconciler, which has all of the uses of this method, we have a mix of calls to ObjectSet.AddSetForType and looping over a map and calling Add on objects individually, depending on whether the type had already been added. I added an easy-to-miss bug adding UIButtons here - my buttons blew away any buttons that were added by the Tiltfile. (similarly, if we add ToggleButton to typesWithTiltfileBuiltins, any ToggleButtons from the Tiltfile will get deleted by the later call to AddSetForType)

Solution

Make AddSetForType merge instead of replace. (which also makes that part of api.go cleaner!)