Custom stepping

[LeonMatthes]

As a smalltalk programmer, I want to be able to attach a custom stepping script to any Morph in order to be able to create custom behavior for slide objects.

Acceptance criteria:

• [x] An option to attach a script to any presentation object exists
• [x] I can enter code into a text field to define the behavior of stepIn:
• [x] The script is checked for syntax errors and does not allow saving when the syntax is incorrect
• [x] Whenever the slide containing the Morph is open the script is executed in regular intervals
• [x] the slide the morph is on is provided as the parameter to the script

Note: This should allow for easy implementation of #72 First concrete step to tackle #71

Estimated effort: 13

[LeonMatthes]

Please note that this user story has many acceptance criteria. This does not necessarily mean that this should be a lot of work to implement. I just wanted to be specific because this is the first step to implement the scripting API, which most importantly needs more specificity.

[LeonMatthes]

New estimated effort: 8