Items: Create new item out of templates

[t-moe]

It would be nice if we could export a new item out of a template.

The new item should have a user definable type and icon, Also the input and outputs should be somehow configurable and the user should be able to configure the newly created template based item.

Implementation Ideas:

• The current Item templates can be "edited" in a own item scene aka special editing scene. When doing so, the user can also drop Input and Output Items on the scene, and connect those. The Input and Output Items will then be used to generate the inputs & outputs connectors of the generated item.
• When editing the template in this special editing scene the user can also add properties which should then be configurable by the generated item's widget.
• Also the user should be able to define new actions for the generated item, which will be a programmatic sequence of the available actions of the items in the template.