JDK + language level autodetection (and other improvements)

I noticed that, aside from some functionality for Android projects (which I don't use) the plugin doesn't make an effort to detect what JDK version and language level the sbt projects are using (instead it derives these from the JVM that sbt runs under, and sticks them onto the IDEA project). 

With this change, I look at `javacOptions` in each sbt project's scope, and if `-target` is set, I set the _SDK_ (specifically, the JDK library to use) and the _language level_ to that version. If, further, `-source` is also set, then I set the _language level_ to the version coming after `-source`.

Other improvements are minor source code rearrangements, things like that, but also a fix which allows slashes in ivy module names to be used (previously, they weren't normalized to `_` and the plugin would crash when it couldn't find the XML file with a slash in it, that it supposedly generated for a library whose name contained slashes).