Split runtime repository into rt and rt-closed repositories

This is in support of moving portions of the JavaFX runtime (javafx-ui-controls, initially) into an open source project of OpenJDK.

The following needs to be done:

1) rename the existing "runtime" repo to "rt-closed"
2) Create a new, empty "rt" repo
3) Copy the javafx-ui-controls directory from rt-closed to rt
4) hg rm javafx-ui-controls directory in rt-closed
5) Fix the top-level build script and any other script that refers to runtime to now refer to rt-closed

More details as available.