Added read only mode to NodeUIs and NodeEditor.

Reimplemented ExpressionNodeUI behaviour using just StandardNodeUI and a custom PlugValueWidget - this avoids having to reproduce the read-only logic and should mean the Expression ui picks up new features automatically in the future. Doing the same for ParameterisedHolderNodeUI might make sense too but would be too involved for this pull request.

Fixes #414.
Fixed read only MultiLineTextWidget bugs.

The style wasn't taking a hold because it needed a repolish. The widget was still receiving drops instead of ignoring them.
Merge pull request #496 from johnhaddon/readOnlyNodeEditor

Read only node editor