Only show plugins which are actually associated with the file from the "open with...

Only show plugins which are actually associated with the file from the "open with" menu. (plugins registered with * in the viewers.config are always shown)
Shows everything if the file doesnt have a extension.
Also fix the plugin name showing the whole path instead of just the name.

Previous commit should have said "Delete your current /.rockbox/rocks folder because the plugins' config/extra files have moved and to save you hassles"

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@14215 a1c6a512-1295-4272-9138-f99709370657