Fixed threading issue in GafferImage::Display node.

Access to the tiles in the display driver is now protected by a mutex.
Moved renaming code from PlugWidget to LabelPlugValueWidget.

This makes renaming available for promoted children of CompoundDataPlugs, the most common example of which is plugs promoted from the Attributes and Options nodes in GafferScene.
Merge pull request #715 from johnhaddon/plugRenaming

Moved renaming code from PlugWidget to LabelPlugValueWidget.
Improved StandardGraphLayout node auto-positioning.

It used assume that the tangent at each end of a connection were opposing - basically assuming that all connections were either horizontal or vertical. By using both endpoint tangents we can now cope with vertical->horizontal and horizontal->vertical connections.
Moved scene node filter plugs to the right hand side of the node.

This makes for tidier graphs.
Merge pull request #716 from johnhaddon/filterPlugPosition

Filter plug position
Merge pull request #714 from johnhaddon/displayThreading

Fixed threading issue in GafferImage::Display node.