This fixes https://github.com/Martchus/tageditor/issues/50.
When keeping the QJSEngine tied to the main thread the garbage
collector will run on the main thread's event loop. This leads to
crashes when trying to allocate memory within the engine from
another thread (`QV4::PersistentValueStorage::allocate()`).
The Qt documentation does not mention that the garbage collector
might run on the event loop of the thread tied to the JSEngine.
I expected it only to run after or before allocations/deletions
within the thread calling the engine's methods. There is already
an issue regarding the lack of documentation:
https://bugreports.qt.io/browse/QTBUG-57227
I found no way to obtain the QThread object for a thread started
with Qt Concurrent. The possibility I found was calling
`QThread::currentThread()` from the concurrent thread once it has
already been started. However, when the concurrent thread has
been started it might already be too late to move the engine.
Adding further synchronization to solve this is an overkill so
I resorted to using QThread directly.
Martchus