Small refactoring in PlayPlugin, cleaning up names.
Rely on the text editor listener for property changes.

Removed additional property change listener, relying on the framework for property
changes. As a side-effect, our editors correctly update themselves when general
editor properties have changed (like "Show whitespace").
Small refactoring in per-project preferences.
Remove AutoHashMap

There's already support for default values in the library HashMap implementation.
Removed unused `sourceFiles` map.

This may look dangerous, but it was a private map, never read. Everything seems
to work fine after removal.