Support for multiple expressions

[dan283]

While some expressions work well for some shapes like the eye network, some other expressions are preferable for other shapes like the jaw network or the neck rig.

• [x] Create an enum dropdown in advanced mode, for expressions to choose from.
• [ ] Create a string input to allow for mass-recomputation of expressions

[dan283]

While the enum thing is done, it might be a good place to add some variables I'll need later, like the shape negate one.