Close ScalaPresentationCompilers which weren't used for specified time

In the case when someone have open a lot of big projects and from various reasons there are created presentation compilers for many of them, they can consume a lot of memory (e.g. each of them can use tens megabytes of RAM or even more). Therefore it'd be better to have possibility to close unused presentation compiler, when given project still exists in a workspace, but user doesn't edit its code. Thanks to that GC will be able to free memory used by them.
 
Each PresentationCompilerProxy has some kind of activity listener which contains a timer checking periodically last activity time.
User can choose frequency in preferences (disable, set amount of minutes, decide whether open editors should be taken into account).
When this feature is disabled or ScalaPresentationCompiler is already closed, an instance of timer is stopped and freed, and also PropertyChangeListener is removed from PreferenceStore.
 
Unfortunately sometimes memory can't be freed, when there are still open editors, and user chose an option to ignore open editors. The reason is that a reference of a presentation compiler is still used somewhere. Investigations performed using the profiler and also empirical ones showed, this problem is related to implicits highlighting. Added code contains adequate comment related to this issue.
 
Also:
- fixed memory leak - ScalaProjects weren't freed due to a listener which wasn't removed
- updated license links
 
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I think it would be simpler to have a single instance of the killer task, instead of re-creating it each time. The delay interval is constant for all tasks, so it can stay as a member of the enclosing class, and makes this whole class be stateless.