Implemented Xflow operator overloading.

There can now be several operators with the same name.
Operators with matching input types is choosen.
If no operator matches, a detailed list of all operators
Merge branch 'xflow-overloading' into develop
Added an option for movement aware click handling. When TRUE a click event will not be triggered if the mouse has moved more than 4 pixels between the mousedown and mouseup events. Default is FALSE.
Added a function to create a XML3D.math.bbox out of a XML3DBox
Use scp for upload
Started implemented asynchronous Xflow operators. Everything untested.
Added basic (kinda hacked) support for asynchronous Xflow operators.
Implemented proper checks for asynchronous operator execution.
Added noAlloc option for Xflow operator output.

If noAlloc is enabled, no output buffer is allocated and passed to the operator.
Instead, a object with the member "assign" is passed.
The operator can allocate its own buffer/image and assign it via
[output argument].assign = [buffer / image];

See the xflow.bufferSelect operator for an example.
Some (possible) bugfix for asynchronous operators in the <shader> element.
Some fixes related to notifications of initial async output.

- On first async output a structure change event should be send
- Don't set nodes of async executure to processed.