CHOP-763: Add LLView::TemporaryDrilldownFunc to support UI injection.

Instead of unconditionally calling LLView::pointInView(),
LLView::visibleAndContains() now consults a class-static boost::function
called sDrilldown -- which is initialized to LLView::pointInView().
Introduce LLView::TemporaryDrilldownFunc, instantiated with a callable whose
signature is compatible with LLView::pointInView(). This replaces sDrilldown,
but only for the life of the TemporaryDrilldownFunc object.
Introduce llview::TargetEvent, an object intended to serve as a
TemporaryDrilldownFunc callable. Construct it with a desired target LLView*
and pass it to TemporaryDrilldownFunc. When called with each candidate child
LLView*, instead of selecting the one containing the particular (x, y) point,
it selects the one that will lead to the ultimate desired target LLView*.
Add optional 'recur' param to LLView::childFromPoint(); default is current
one-level behavior. But when you pass recur=true, it should return the
frontmost visible leaf LLView containing the passed (x, y) point.