Moved Plug binding details to anonymous namespaces.
Fixed deadlocks if computes spawn their own threads.

We need to release the GIL on the thread which requests the compute, so that if the compute spawns threads, they can enter into Python.

The only example we have of this right now is the Instancer when getting the bounds, but I've fixed all the ValuePlug::getValue() bindings too for good measure.
Added GafferScene::PathMatcher::prune() method.

This is equivalent to removePath(), but additionally removes all descendant paths.
Added GafferScene::PathMatcher::isEmpty() method.

Fixes #1076.
Merge pull request #1091 from johnhaddon/getValueGILManagement

Fixed deadlocks if computes spawn their own threads
fixed SubTree sets problem where root path has no leading slash
Fixed PathMatcher::prune( std::string ).
Merge pull request #1096 from davidsminor/subtreeSets

fixed SubTree sets problem where root path has no leading slash
Merge pull request #1095 from johnhaddon/pathMatcherAPI

PathMatcher API additions
Added GafferBindings::NodeClass() overload for suppressing init.

Not all nodes have a constructor taking a name, so we need to be able to optionally define the constructors manually.
Simplified Viewer implementation.

It's easier to store everything as attributes on the View instances - using dictionaries indexed by View doesn't extend very well if we want to store more things.
Added some tool icons.
Added Tool base class.

This will be used to help implement interactive tools inside the Viewer.
Added GafferSceneUI::SelectionTool.

This is refactored out of SceneView into the new Tool framework.
Tool menu in Viewer. Squash with previous commit.
Boilerplate for CropWindowTool.
Moved shutter() and transform() from RendererAlgo.h to SceneAlgo.h.

This is binary compatible, but source incompatible.
Added camera() methods to SceneAlgo.h.

These return cameras from the scene with all globals such as with all globals settings such as crop, resolution, overscan etc applied. Updated RendererAlgo to use these methods rather than duplicate logic.
Removed GLWidget minimum size.
Added missing virtual prefix in SelectionTool gadget.