Instead of calling `BKE_nlastrip_new()` (which, due to backward compat
reasons automatically picks a slot), the Push Down operator now calls
`BKE_nlastrip_new_for_slot()`, which explicitly assigns the given slot.
On top of that, the frame range of the slot is used to set the strip's
frame range (instead of the range of the entire Action).
Instead of using direct property manipulation to enter tweak mode, use
the regular `animrig::assign_…` functions. When used in the right order,
as introduced in this commit, the "disallow assigning an Action in tweak
mode" logic can just stay simple as it is.
