til_{tap,stream}: introduce til_tap_t.inactive

When a driving tap becomes inactive, til_stream_tap() should be
able to notice and replace the driver.

An example driving tap becoming inactive would be a GNU Rocket
track that once had keys in it, but then had them all deleted.
This should set the inactive flag so the tap's automation can
take over.  This gives the user at the Rocket editor the ability
to both take over from the tap automation and surrender control
back, by populating vs. emptying the respective track.

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