Enhance GraphicsEditorWidget

[rchristie]

A number of Graphics attributes, some quite key, are not yet editable in the GraphicsEditorWidget, including:

• [x] DomainType
• [x] Native tessellation field
• [x] Several point sampling attributes (sample density field, sample location)
• [x] Glyph offset
• [x] Glyph signed scale field (used for eigenvalues in strain visualisation).
• [x] Glyph repeat mode (incl. mirror used in strain visualisation).
• [x] Selected material
• [x] Render line width and point size
• [x] Label offset and fixed text
• [x] Contour sequences -- currently only support fixed values.
• [x] SelectMode

Also:

• [x] BoundaryMode enum replaces and generalises the exterior flag. This is quite an important one for making several current SPARC visualisations e.g. boundaries of groups.

[hsorby]

The boundary mode enum appears to already be present.

[rchristie]

Yes, I got Kay to put BoundaryMode in early with her enumeration chooser widget as it was one of the main drivers for it.

[rchristie]

Some items moved to #47.