5.10.5 -- Fix issue with @imports in .inc.css files

- The CSS include files (i.e. the *.inc.css files in the project or in
  dependency Jar files) are concatenated to form the main.css file
  that's included in the <head> by default. This concatenation is a
  problem, though, because @import rules must appear before all other
  rules in the stylesheet. This fix hoists @import rules to the top of
  the generated CSS stylesheet, in dependency order, and preserving
  leading comments that might be in the .inc.css files.