Introduce an incremental garbage collector for QML

Rough outline of required steps:

1. Prepare tests for introduction of incremental gc ✔
2. Introduce incremental mark phase ✔
3. Introduce incremental sweep phase ✔
4. Make feature (build-time) configurable †
5. Validate performance (of the engine), optimize if necessary
6. Investigate if we need to change when to invoke the gc if its in incremental mode ✔
7. Document knobs which we might expose

† It's no longer possible to make the whole feature optional – we moved away from black allocations while the gc is running, so now the write barrier is vital even in non-incremental mode. It might still make sense to expose knobs to disable incremenatl gc.