Bug 899792 - Establish precompile tier and move IPDL and WebIDL build rules; r=glandium

A new build tier is introduced: precompile. The intention is to replace
the export subtier with something better and nonrecursive. IPDL and
WebIDL code generation have been moved into this new tier and are
performed concurrently. This appears to reduce build times due to fewer
"single threaded" build actions.

The subtier tracking UI has been updated to reflect operations that can
occur in parallel. As subtiers are started, they turn yellow. They can
then complete in any order.

Nonrecursive targets have been added to preserve the IPDL and WebIDL
build targets such that developers can type |make| from leaf directories
like they have always done.