Images which are loaded by CSS should be cached.

Example 1:
- create 2 CSS classes (a and b) with the same -fx-background-image
- create a node with CSS class a
- create a node with CSS class b
- image is loaded 2 times to the memory
-> image should loaded once

More critically:
Example 2:
- create 1 CSS class with -fx-background-image
- create 10 nodes with this CSS class
- image is loaded 10 times to the memory