Move timer and tip out of button code,
now the only requests a tooltip,
passing a creation callback to run.
Needed for manipulators in 2.8,
also helps de-duplicate logic - since we never want
multiple tool-tips showing at once.
This makes code closer to id_override/assent-engine ones, which
introduce a new type of linked data, and hence reserve
ID_IS_LINKED_DATABLOCK to real linked datablocks.
