Fix crash when dynamically unloading a MouseArea

If a MouseArea has hoverEnabled = true and is in the hover state,
setting hoverEnabled = false and then unloading that comopnent
would cause a crash.

Task-number: QTBUG-30962

Change-Id: I4b71851092c7fe178fed85b672e4c206458b46f9
Merge github.com:suitabletech/qtdeclarative into st