Added an undo stack, so you can do more than 1 undo.
Added erosion operation.
Added new drawing tools: line, rectangle and circle.
Code cleanups, less bloat.
Editor window has a status line at the bottom.
Grid size and editor-window size are independent.
Reading of 8bit PNG files with transparency should work.
When selecting the shown tile in the memory-window, the selection centers on the mouse cursor.
Zooming can now be done is discrete steps, instead of having fixed zoom values.