Add note about global gitignore in our .gitignore files.

See discussions in I7ca6a7fca, I66f1fd9c1 and I9efffccfa

Change-Id: Ice44d016697b359d92ebaa5a1b8348b9d3afc59e
Make parent POMs bills-of-material and unbundle ASM

ASM is no longer rebased, so we now declare a dependency rather than
bundling it inside gwt-dev.
The POMs are now also BOMs so you can make sure you have everything with
the same version (even transitive dependencies from 3rd-party libs) with
just a dependencyManagement declaration with scope=import.

Change-Id: Ic1a81df9cef47bf1fefcd3962f64400c4afc43db