Use plain Scala OSGi bundles to build the IDE.

* toolchain is used only to build sbt
* all Scala-related dependencies are resolved through Maven
* bundles not needed for execution (scala-swing, continuations, various source attachments) are added to sdt.core
* features need an extra step, because in 2.11 we need to ship two extra bundles (xml and parser combinators). This is done through the new -Pset-features profile in build-all.sh

As a bonus, all Scala bundles have correct source attachments now, so plugin
development should be much easier.

Fixed #1001889.