Patch by Ovy:

Simplify, speed up AtomicString using QT4 thread safe refcounting.

Simplifications: no more lazy deletes, checking for main thread.
Speed-ups: refcounting is generally outside the store lock now; some methods became trivial and could be inlined easily.

@Ovy: moving the ~AtomicString() definition to the cpp file fixed the linker problem for me.