Added 'auto expand' mode to the scene viewer.

This is implemented with a "minimumExpansionDepth" plug on the SceneView class, which specifies a number of levels to always expand by, regardless of any other settings. Currently this is exposed simply as an on/off menu item in the viewer toolbar as Ben originally requested it. Also added menu items for the existing expand/collapse selection functionality, which should make the keyboard shortcuts a bit more discoverable even if the menu items aren't that convenient in themselves.

This continues the accumulation of nonsense into the SceneProcedural in support of viewer functionality, so I added a todo to remove all the nonsense when we find the time to have the viewer do its own conversion of the scene to OpenGL.

Fixes #163
Tidied up graphics file.

Some bizarre "spiro path" effects had snuck in, which made editing the paths very confusing, even if they looked alright to the eye.
Replaced duplicate code in SceneView with call to ScenePlug::stringToPath().
Added Shift+Down shortcut for full expansion in the scene viewer.

The logic for this is a little complicated, and the chances are high that we'll refactor this code in the future when the viewer does its own scene management, so I also added test cases to check that things work as expected in the new code and will continue to in the future. This also fixes one bug, where leaf level items were being marked as expanded, meaning that they'd require two presses of the Up key to get them to collapse.

Fixes #556.
Merge pull request #671 from johnhaddon/viewerExpansion

Viewer expansion