Install the NP-complete level sets alongside the regular levels.
Remove now-unused worldAABB.
Avoid a segfault: skip drawing empty text.
Remove unused ScrollArea::m_canvas, avoiding a memory leak.
Use local variables instead of heap allocation.  Avoids memory leak.
Clean up includes.
Add const qualifiers to methods.  Add trivial test cases.  Fix bug in suffix testing.
Change Levels::addPath() to void; return value is unused and uninformative.