Unify kepler and luna branches as different profiles under the same build

This commit “merges” the platform/luna branch into master, using separate
source folders for the source-incompatible differences between the two platforms.
In this way we don’t need to regularly merge master into Luna, and it makes it
easier to use Luna for developing the IDE itself.

* Changes that proved to be source-compatible, but only added in Luna, I
  merged them as-is (without relying on a separate source dir for those)
* jdt.core versions in the manifest file are now replaced when they are
  copied using the maven-antrun plugin. The substitution is performed automatically,
  but the syntax is slightly different than for pure maven resources. It’s
  not ideal but there is no way to both substitute variables and rename a given
  file.
* I made src-luna be the default in project definitions.

After merge:

[ ] Switch nightlies to use master for luna
[ ] Update developer docs