Define SVG icons in the CSS, add activity-stop.svg icon

We need the icons next to the CSS to reference them with a relative
url.
Improve API to colorize an icon

Move the details to this library.  Usage example:

    icons.colorize(myButton, ["#FF0000", "#00FF00"]);
Merge pull request #1 from manuq/api-improvement

Api improvement
Remove pointer cursor from buttons

Is not consistent with gtk activities
Merge pull request #2 from manuq/fixcursor

Remove pointer cursor from buttons
Fix variables declaration

Otherwise they mess the context after loading the module.  Can be
tested in the inspector console doing:

    var icons = require("sugar-html-graphics/icons");

Objects like 'strokeColor' will not be available anymore after this
patch.
Merge pull request #3 from manuq/fixvars

Fix variables declaration