Level file format has been changed.

Previously, the level file format consisted of the name of the
image to use for the background tile, followed by lines representing
bricks, with each line having 5 fields: row, column, boolean for
brick breakability and type of brick.

The format has been reduced to simply row, column and color name.
The brick's breakability and hit points are determined by their
color.
Implementation of editor's full basic functionality.

Editor can now specify a background tile and save into a file.