View grouping in thumbnails sidebar

[lrecknagel]

Addresses #87

• I implemented a first version of a view grouping approach in the thumbnails sidebar.
• Its triggered by a custom view.properties entry:

properties {
    viewGroup "name"
}

• If no view has this property set the current default behaviour is rendered.

• Example

container containerId "containerName" {
  include *
  autoLayout
  properties {
    viewGroup "customGroup"
  }
}

• Every other view with the same viewGroup prop will appear in the same viewGroup / section.

I am happy and open for comments & feedback.
Thanks to everyone who make that project possible!

[lrecknagel]

@simonbrowndotje Let me know if it needs certain changes. Would love to push this further :)