use relative file paths for plugin project css files

When loading a plugin project, the file paths to css resources where
replaced with absolute URI's. As resources belong to the project, the
path should be relative.

Change-Id: I3479e80dbecd440fb74763c1b5a1be484b7f3b11
Signed-off-by: Carsten Hiesserich <carsten.hie@gmail.com>
4 files changed