qt/qml: allow certain functions to be called in qml

Usually, a standard QML application does not need to access these functions. However, when the application becomes complex enough, it becomes useful to call some of the functions that are not invokable.

For example, an application should use FocusScope, because it sets the focus scope flag internally. However, this approach does not fit to certain situations. Such that when a Rectangle needs to be a focus scope, currently there is no way to make it a focus scope except adding a new FocusScope as its parent.

I also removed the property adjustment in mouse event filter. It is not very good idea for a filter to change target object properties. Accepted mouse buttons are now set in QML.