load only one stylesheet for all modes instead of three

Instead of three stylesheets for 'all', 'screen' and 'print' modes, they are all loaded into a single stylesheet by wrapping all screen styles in a "@media screen {}" and all print styles in a "@media print {}". The 'all' mode is not wrapped in anything.
Potential issues with existing CSS: If any of your screen or print CSS files already contain any "@media" syntax, the CSS will probably break. In that case please add any CSS with "@media" in it to the 'all' mode instead!

Also, the 'rtl' mode is hereby deprecated. Please just prefix any RTL styles within your normal CSS files with "[dir=rtl]". This also fixes that RTL styles cannot be added for 'all' or 'print' modes.