block actor support arbitrary thread context

[guruofquality]

From @guruofquality on August 19, 2014 7:24

In an effort to be more versatile.. and to allow blocks to be driven by systems that spawn their own threads... come up with a way to "handle" a block's actor from an arbitrary thread context. One really good use case would be using the Qt main applicaiton thread to run the block so there would not be any thread issues between widget access and other block actions.

Copied from original issue: pothosware/pothos-library#66