Improve the scripting environment

[jkarder]

Some improvements can be made:

• IEnumerable<T> extension methods should be usable with the variables collection.
• If an INamedItem is stored in the variables collection, the name of the item can automatically become the name of the variable.
• Renaming variables should be possible by pressing F2.
• Removing variables should be faster.
• Selecting multiple variables in the VariableStoreView should be possible.
• Scrolling inside the code editor should be possible when executing scripts.