o.c.opibuilder: Fix race condition in ResourceUtil URL Loader

Used to check cache, then later read from cache... but cache entry could
expire in between those calls.

Using general Eclipse 'Job' instead of JFace & PlatformUI calls.
Unclear why Job/Thread in the first place, but leaving that in place
because no time to check all call paths.
Merge pull request #372 from ControlSystemStudio/bug370

o.c.opibuilder: Fix race condition in ResourceUtil URL Loader