Polymorphic load-media entry point

Playback starts from a lot of places: a search row, a library row, a playlist, an album, a shared link, a restored session. Each has its own item type and its own idea of what should be queued next. Given one entry point per caller, any policy that must hold for all of them will be missing from at least one.

Collapse them into a single function with two parameters that do the varying: