Add control over handle connectable properties

[philippjfr]

This exposes the following new flags on nodes:

Flag	Default	Controls
input_connectable	True	Whether input handles are connectable at all
input_connectable_start	True	Whether edges can start from input handles
input_connectable_end	True	Whether edges can end at input handles
output_connectable	True	Whether output handles are connectable at all
output_connectable_start	True	Whether edges can start from output handles
output_connectable_end	True	Whether edges can end at output handles