Improvements for expanding SceneCaches in Houdini

This request address the first round of user testing as relates to expanding SceneCaches in Houdini. Specifically:

 - Expanded nodes are now connected to the subnet indirect inputs
 - Expanded nodes are placed using Houdini's layout automation
 - File parms channel reference their parent
 - Path parms channel reference their parent (when appropriate)
 - Expanding and SOP cooking are now interrupt-able using the escape key

Also fixed a crash when middle clicking SOPs that failed to cook