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.
Reviewer comments:

- added Scala compiler sources
- added xml and combinator versions only in the 2.11 profile
- removed useless dependency on HasLogger
- removed exported dependencies from sdt.core project files
- removed superfluous version numbers in dependencies
- fixed runtime library for build-tools to always be 2.10
Removed old and no longer used CHANGELOG file
Merge pull request #576 from dotta/transient/remove-changelog

Removed old and no longer used CHANGELOG file
`ScalaClassFileEditor` should always hold a `ScalaClassFile`

There doesn't seem to be a good reason for not creating a `ScalaClassFile` when
source attachments cannot be located.

Doing so has two nice consequences:

1) When sources are manually attached to a Scala JAR, and a classfile is opened
   in an editor, the source is now correctly syntactically highlighted. This
   basically fix half of ticket Re #1001926 (the other half is triggering
   semantic highlighting).

2) `ScalaClassFileEditor` instances are now ensured to be always created with
   an underlying `ScalaClassFile`. This is an important fact, as
   `ScalaClassFileEditor` assumes it (in fact, the `ClassCastException`
   reported in ticket Re #1001925 was happening because this implicit contract
   was broken, i.e., it was possible that a `ScalaClassFileEditor` could be
   created with a (Java) `ClassFile` when no source attachment was found.

By the way, the `ClassCastException` issue mentioned in point 2) could be
reproduced by copying a **Scala** classfile in the root of a project, and then
trying to opening (by double-clicking on it from the Package Explorer view)
would result in an error prompted to the user.

Fixes #1001925
Merge pull request #575 from dotta/issue/semantic-highlight-class-cast-exception-1001925-bis

`ScalaClassFileEditor` should always hold a `ScalaClassFile`
Merge pull request #565 from dragos/improvement/scala-osgi

Switch to vanilla Scala OSGi bundles