Force generate R.java

The point is that there is no sense in "Force generate R.java" action in
IDEA 12, because we never generate full R.java file in the project. We
generate it only during project compilation in temp directory which is not
visible for user. Also we generate special stub R class in "gen" root, but
it doesn't contain any data, we only need it to provide completion/resolve
in the IDE. In previous EAPs of IDEA 12 the action "Force generate..."
regenerated the stub empty R file I mentioned, which is unuseful because
stub R file always exists.