Add a locator filter that runs arbitrary command in current project's build directory

We already have a locator filter "!" that runs an arbitrary command in the directory of the "current document". That is nice for triggering custom commands on the source code, but it would be nice to have a way to trigger custom commands on the build as way (like running custom build commands).

I'd suggest a new locator filter "!!" that does the same as "!", but in the "current" project's build directory instead. Code should be mostly shared except for getting the working directory (and id, name, & description).