panel: Silence allocation warning for external plugins (V2)

This strives to impose a minimum size on external plugins only as long as necessary to silence allocation warnings, thus avoiding side effects noted in !84 (comment 51278).

There does not seem to be a reliable upper bound on the time interval during which this minimum size is required, hence the use of a timeout which is restarted as long as one of the external plugins requires a reallocation on its own (this is typically needed at panel startup, when several plugins are loading in parallel).

This should eliminate most allocation warnings (unless you are on a really slow or overloaded system), without impacting the size of the plugins beyond loading.

Fixes: #590 (closed)

Related: !84 (merged)