Cleanup and reorganize core and canvas helpers

Move some of the "core" and "canvas" utils in `helpers.core.js` and `helpers.canvas.js` and introduce the new `isNullOrUndef` and `isObject` helpers. Deprecate `indexOf` and rename `drawRoundedRectangle` to `roundedRect` which now creates a simple `rect` path if radius is 0. Write missing unit tests for the moved helpers.