Fixed LayersModel not handling underscores in layer names correctly.
Added Ctrl= and = shortcuts for zooming in.
Created an application icon (based on Tiled's icon).
dist.tcl places WorldEd/ as a subdirectory of TileZed/.

This is to enforce using the same version of WorldEd as TileZed.
This is needed because both programs share the ~./TileZed directory and
some of the files in that directory are managed only by TileZed.
Added newline at the end of mapcomposite.cpp (git doesn't like it).