Adds validation state feedback to nodes

[tatatupi]

The idea is to have a NodeValidationState member object in the NodeDelegateModel class, that defaults to a valid and empty state message. The API allows for updates to be made accordingly, and the UI will respond automatically.

UI will show a node red (error) or yellow (warning). A tooltip is added with the state message. And a small icon for information is displayed as a badge, to help the user access the tooltip. I added a screenshot of the calculator example to help discussions. in the example, the tooltip for the Warning state is displayed.

Key Changes

• Introduces NodeValidationState struct class.
• Getter and setter in NodeDelegateModel for validation.
• DataFlowGraphModel handles NodeRole::ValidationState updates.
• Painting logic reflects invalid states in DefaultNodePainter.
• Tooltip shows validation message in NodeGraphicsObject.
• Calculator example demonstrates usage.

Solves #226
Solves #75

P.S.: some changes happened automatically when saving files, due to autoformatting using nodeeditor clang format style.

[tatatupi]

I also changed the linux-gcc qt version to 6, so that QtGui/QUndoCommand include could work