Avoid calling gc in QQmlEngine destructor.

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>