Workflow labels management UI

The labels designer module should provide a management UI that lets the user manage labels so that they can be assigned to workflows.

## Label management
When creating a label, the user should be able to provide the following details:

- Name
- Color

The management UI should be invoked as a modal from the top-right menu item between Activity Definitions and Settings (see screenshot:1)

The UI should allow the user to add, edit and remove labels.

## Label assignment
The user should be able to assign labels to the current workflow definition from the Properties panel (see screenshot:2)