webapp: new APIs

* added createTextLabel, setPosition, setParent
* added canvasSetStrokeColor, canvasSetLineWidth, canvasSetFillColor
* add width,height parameters to createCanvas - optional
* changed generateDropletPalette() to create multiple categories if categoryInfo is passed in
* leveraged this new change for webapp by creating a separate Canvas category in droplet
A brief comment on each of these methods would be helpful, at least listing which properties on "opts" are being used.