effects/private: Merge two state machines into one with substates

Having one state machine instead of two somehow simplifies the code and
makes it less fragile.

The new property `substate`, which replaces `thumbSource.state` still
supports imperative assignments that are so important for Drag&Drop,
and still triggers state updates. However, now a substate may not
become an active state: that behavior didn't make sense in previous
iterations, and has led to a glitch where a window won't resize during
return-to-initial animation. The glitch happened because two state
transitions were starting almost-simultaneously (and by Murphy's law in
the worst possible order, of course), and both animations were playing,
but one animation was supposed to to a property that is managed by the
other. But since QtQuick animations cache their from/to values before
they start, the dependent transition essentially didn't animate at all.

(cherry picked from commit 9897afa5)