Fix Collection Browser auto-expand after search expanding too little

Underlying model is asynchronous, we need to cope with that. Do it
using a RAII self-destructing object AutoExpander.

As a side-effect, Amarok no longer expands collection after start in
split collection mode. If this is desired, we may expand them
explicitly upon start.

BUG: 335217
FIXED-IN: 2.9
CCBUG: 300557