Enhanced set dimensions

Added ability to set css & backstore dimensions separately 
Introduced options param to the dimensions setters, options.backstoreOnly & options.cssOnly

When backstoreOnly set to true, only the value of the canvas element width & height will be changed.
When cssOnly set to true, only the css width & height of the involved DOM elements will be changed,
and you should include the unit of measure (px/%/em) with the passed value (eg '100px' '100%') and not just 100
Actually, looking closer at this... I don't like use of `''` as no-op at all. And double inline statement via comma operator? Ouch :)

Let's just do a good old:

```js
if (options.cssOnly) {
  this.setBac...
  cssValue += ...
}
```