UI and definitions for more specific action special argument types

Basically this:

![screenshot](https://cloud.githubusercontent.com/assets/94112/5211483/52c33566-7598-11e4-876d-043032c1ce18.png)

There's also a ComboBox implementation for arguments with several discrete mutually-exclusive values.

Most of the plumbing was already there, as `custom_values` and `custom_flags`; this is mostly UI.

I didn't go through the entire ZDoom configuration updating special definitions yet, and there could probably be more types (angle, opacity, speed, delay, lock...), but I want to make sure this is all kosher before I go too much further with it  :)