Implemented custom pointer icons for all drag and drop operations.
Improved (hopefully) icons used for drag and drop.
Fixed Shader node bindings to use DependencyNodeClass.
Fixed texture shader to work with GLSL version 120.
Reverted non-gui ExecutableRender::execute() to block until the render is complete.

Not having this was preventing the test and screengrab apps working correctly.

Fixes #353.
Refactored Shader classes.

- Removed Shader::shader() and Shader::shaderHash() virtual methods, replacing them with Shader::parameterHash() and Shader::parameterValue() virtual methods. This simplifies the task of implementing derived classes.
- Made Shader::state() return a const value, making it possible to cache values in the future.
- Fixed hashing of RenderManShader coshader array parameters.

This refactoring should make it easier to implement the enabledPlug() and correspondingInput() stuff required by #327.
Fixed overzealous drag begin behaviour on LabelPlugValueWidget.
Fixed root cause of drags being started with no buttons down.
Fixed over-zealous NameLabel dragBegin.
Implemented enabling/disabling for shader nodes.

By default disabled shaders behave as if their output connections didn't exist. RenderMan shaders may change this by defining a "primaryInput" annotation nam
ing an input coshader parameter - this will be used to make the disabled shader act as a pass-through.

Fixes #327.
Merge pull request #386 from johnhaddon/dragDropPointers

Custom pointers for drag and drop operations
Merge pull request #395 from johnhaddon/disabledShaders

Enabling/disabling for shader nodes
Fixed packaging of RenderMan display drivers and procedurals.
Fixed 'NoneType' object has no attribute '_leaveSignal' error on mac.

This was presenting itself on shutdown after the node menu had been shown. I simplified the menu code down to isolate the problem but couldn't see anything wrong there in terms of ownership or cycles, so just did the pragmatic thing and added a test for None-ness in the leave event handler.
Packaged IECore::PointDistribution tile set.