Blog post introducing the Scala Installations feature.

This documents https://github.com/scala-ide/scala-ide/pull/731

The PR is going out early (see tasks below) for review purposes : at .2.5K words, it's a hefty read.

- [ ] Add a banner on the old multiple versions blog posts saying the present post supersedes them.
I would add a phrase here. Once you say `that's it!` you should explain what it means: your project is going to be built with that version of Scala, etc. Otherwise, you continue with what is *not* happening (the project settings), which is of secondary interest.