Cleaned up MapRenderer use of mAbortDrawing.
Fixed several cases where "return 0" was used instead of "return QString()".
Made 2 global functions into methods of FurnitureGroups.

The only reason they were global is because I thought the header would have
had to include simplefile.h, but that isn't so because only references to
SimpleFileBlock are used.
Added 'const' to various arguments where GCC complained (but not MSVC).
Added parentheses in an "if" expression in BaseObjectTool::mouseMoveEvent.
Removed an unused variable.
Merge branch 'zomboid' into tile-image