Made the variable `$<folder>-prefix` be actually used, as for today it is simply ignored. The changes are the following:
	1. Modified the `$<folder>-prefix` variable to default to the sprite folder name;
	2. Changed `$<folder>-sprite-base-class` to make use of the prefix variable;
	3. Replaced the default values for the prefix arguments on the sprite instances mixins to be the value of the prefix variable.