Implement setLineDash() and getLineDash() in CanvasPathDrawingStyles,
which write/read from the CanvasState object.
This doesn't implement the actual drawing of a dashed line, but at least
sites using the Chart.js library no longer fail with an exception.
Unfortunately the Painter classes don't support dashed/dotted lines
based on segments yet.
Add the CanvasTextDrawingStyles mixin with the textAlign and
textBaseline attributes. Update fill_text in CanvasRenderingContext2D
to move the text rect by the text align and text baseline attributes.
Wrote a simple HTML example to showcase the new features.
