[Feature request] More "custom properties" types

[pgilfernandez]

When working with expressions, the "custom properties" of an object comes to be pretty handy when you want to set up and drive how an expression works. With that in mind it would be great to, apart from "single value" and "two value" properties, it would be nice to be able to create:

• object: to be able to call an object name from your expression and make it settable
• color
• checkbox (true or false)
• string

Thanks!

[rodlie]

Depends on #59