Avoid calling gc in QQmlEngine destructor.
GC may be expensive. GC call in QQmlEngine doesn't have much sense
because V8 will destroy all objects living in a current context.
The only problem is that V8 may decide to not invoke weak callbacks
which may cause a memory leak. To avoid that we track all QObjects that
have JavaScript ownership set and we delete them explicitly.
The change reduce time of destroying QQmlEngine by 75%, which is really
visible in qquicklistmodel test.
Change-Id: I2a3668fd23630669114baee8c241a7ecc4100e33
Reviewed-by: Chris Adams <christopher.adams@nokia.com>