XWIKI-10708: Allow LESS syntax in SSX.

* Add the property "contentType" to the SSX objects
* SSX objects can handle LESS code
* Create Skin and Color Theme references
* Continuing the refactoring of the LESS module to be able to compile LESS resources that are not files
* Lot of tests done locally in addition of unit tests, that is why I allow myself to low down the jacoco requirement, with the ambition to write new tests in the following weeks.