- add text tool
- add arc tool
- move shapes up/down z-order
- zoom
- bug: start but don't finish editing attribute, then change attribute list
- forbid selecting 0 for stroke width on line, polyline
- scroll bars
- file I/O
- undo/redo
- AttributeManager: should listen for changes to selected objects' attributes

                  text line polyline polygon arc rect rrect oval
    stroke width   .    x    x        x       x   x    x     x
    stroke color   .    x    x        x       x   x    x     x
    fill color     x    .    .        x       .   x    x     x
    font           x    .    .        .       .   .    .     .
    alignment      x    .    .        .       .   .    .     .
    corner radius  .    .    .        .       .   .    x     .
    
    edit handles   .    .    x        x       .   .    .     .