Hid Shader enabled plug in the UI.

Fixes #398.
Implemented ganging for CompoundNumericPlugs.

This is provided at the API level by new methods on the CompoundNumericPlug. It is then made available to the user on the popup menu on plug fields, and via a Ctrl-G shortcut when the cursor is in
a plug field.

Fixes #402.
Merge pull request #405 from johnhaddon/ganging

Ganging for CompoundNumericPlugs.
Added menu item for loading renderman shaders from file.

Fixes #125.
Fixed error which could occur when a non-focussed PathChooserDialogue received a click.
Fixed hiding of Menu when using the search box.

Previously, the __menuActionTriggered method was getting called /after/ the menu command had returned. This meant that if the menu item popped up a modal dialogue, the menu wasn't hidden until after the dialogue had returned. Connecting directly to the signal on the menu item, /before/ connecting the command means that __menuActionTriggered is called first and the menu gets hidden at the right moment.
Merge pull request #406 from johnhaddon/loadShaderFromDisk

Menu item for loading a RenderManShader via a file browser