Refactor

Trying to work on performance. Canvas is not perceivably  faster than a
Grid, but moving away from using ColumnDefinitionCount and
RowDefinitionCount towards currentColumnCount and currentRowCount is
still probably a useful change. Temporarily adding the canvas code as
comments, so I can switch back if needed.

Canvas elements don't auto-size, so the layout looks a little different.
That can be fixed if I provide manual sizes to the elements. But if it's
not faster, I won't add more code.
Update startup to be more general.

Eventually this has to work for non-gba files, so I might as well
broaden it's inputs and rename things to be more appropriate. Also
removed the Canvas experiments, since further experimentation has shown
that switching to Canvas / fixed size grids / DrawingVisuals does not
provide any significant performance boost, while it does increase the
code complexity.
Pull the Command Factory out of the MainWindow

MainWindow started getting beefy. The easiest piece to pull out is the
CommandFactory interface. The new implementation requires a few methods
on the MainWindow to become public.
Outline for Maps parser

Added a code-version of the map layout, and added a template for the
parser.