Designer Activity Definitions

[sfmskywalker]

This PR adds a new feature that lets the user create custom activities using the designer.
These activity definitions then become available from the activity toolbox.

Some of the most important features are:

• Users can create new Activity Definitions using the designer.
• Activity Definitions are versioned.
• Workflows that use a custom activity definition are locked to the version of the activity definition at the time it was added to the canvas. This prevents issues when workflow instances are resumed while e.g. some custom activity definition changed in the mean time, which would lead to unpredictable results and errors.

Future updates will include:

• Ability to define input and output.
• An option to automatically update workflow definitions with the latest version of the activity definition being published.

Demo: