"Search" for types with main() methods should use an indexed search

The X10 launch configuration editor provides a "Search" button to bring up a browser that lists type having a main() method.

Currently, this search does a synchronous search over all of the X10 source code in the given project. This could be fairly slow for even medium-sized projects, as it involves parsing every source file, and at least obtaining name bindings (so that it can properly determine the type signatures of methods).

Instead, it should use the search index, once that actually exists.