introduce smarty_function_load_less:

- add CM_Asset_Css_File to be able to render single file
renamed load_less to load_lessFile
remove possiblity to skip adding variables
make _addVariables public
remove CM_Asset_Css_File, use CM_Asset_Css instead
rename file
Merge pull request #1488 from mariansollmann/issue-1488

Add posebility to render single less files