V4: track C++ heap usage for Strings in the MemoryManager

V4: track C++ heap usage for Strings in the MemoryManager

... and do a GC run when it exceeds a threshold. The issue with Strings
is that they hold on to QString instances that store the real content.
However, the GC only sees the light-weight JS handle, and doesn't take
the size of the backing content into account. So it could happen that
big QStrings accumulate in the heap as long as the GC didn't reach its
threshold.

The newly introduced unmanaged heap threshold is upped by a factor of
two when exceeded, and lowered by a factor of 2 when the used heap space
falls below a quarter of the threshold. Also grow the threshold if there
is enough space after running the GC, but another GC run would be
triggered for the next allocation.

There is a special case for Heap::String::append, because this method
will copy the data from the left and right substrings into a new
QString. To track this, append notifies the memory manager directly of
the new length. The pointer to the memory manager is stored in
Heap::String, growing it from 40 bytes to 48 bytes (which makes it still
fit in the same bucket, so no extra memory is allocated).

Task-number: QTBUG-42002
Change-Id: I71313915e593a9908a2b227b0bc4d768e375ee17
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>