This should fix plugin selection once and for all. If no exactly matching plugin is found we now look for the plugin with the most specific matching mime type using a topological sort on the mime tree. Might be overkill for the plugins we have so far, but things get more and more complicated with each new one.

Results are cached now, so it should be even faster than before for mime
types with no exactly matching plugin.

svn path=/trunk/KDE/kdepimlibs/; revision=979872