Don't Execute Paused Node Twice

[brwarner]

We have a subtle bug where the present, paused on node is executed twice, once upon arrival, and once upon leaving.

This is caused by the logic in our UpdateAvailableBranches code. It always adds the current node to the start of every available branch. When advancing along a branch, we execute each node in the its path. Since the current node was previously the last node in the path of a branch, and is now the first node in the next branch, it's executed twice.

Normally, this was not a problem since only OutputPin and Instruction nodes have an implementation of Execute and usually Instruction nodes are not paused at. However, if the user does set Instruction in their Pause On setting on the flow player, they'll see this bug.

The original behavior of adding the present node to the start of each branch is maintained whenever we reset the current cursor, as we presumably want to make sure we execute the node we begin at.

[brwarner]

Fixed a few bugs when setting an Instruction as the start node. Still need to do some more testing as this is a higher risk change.