Made the BMP selection the same as the tile selection.

Having 2 separate selections was potentially confusing, and it is useful
to use the tile selection for both tiles and BMP pixels.
Don't clear the tile selection during Edit > Delete.
BMP selection tools perform copy with Ctrl and affect all BMPs with Shift.

Previously, Ctrl affected all BMPs.
Added BmpEraserTool for erasing pixels in a map's BMP.
Use Bresenham circle algorithm for BMP tool circle brush.
Added "Contents" to TileZed's Help menu and documented the BMP tools.
Don't display the "condition" color for rules with index=0.
Use #define WORLDED to keep code in sync with WorldEd.
Added a HELP! button to BmpToolDialog.