[BUG?] Grammar auto-builds java sources, even when it's not in a source directory

[xoxota99]

I keep a bunch of .g4 files in a "doc" folder of my project, for referring to. I don't intend for those to generate sources. Despite setting the correct "source" folders in my project settings, the ANTLR tool still generates Java source for any grammars it finds, regardless of their location in the project.

[HSorensen]

This should be solved together with #100 by adding a "build-path" preference page that by default only include source folders.