Slate 0.5.0

• Added brush shape selection tool button with a choice between square and round.
• Fixed pen/eraser tool behaviour by using line tool for all drawing/erasing (#39).
• Fixed incorrectly drawn lines (#7).
• Added Hue/Saturation dialog
  • This allows changing the hue, saturation and lightness of the current selection.
    Note that a current selection is required, as this greatly simplifies the code.

A huge thanks to @not-surt for the drawing patches and the introduction of a round brush!