Port away from KMimeTypeTrader

This means that KService::Ptr for parts should be replaced by
KPluginMetaData.

There are some problems, however:
- KonqMainWindow::updateViewModeActions uses KService::actions. As far
  as I can tell,there isn't a replacement for it
- the documentation for KPluginMetaData::serviceTypes states that "This
  is mostly for historical / compatibility purposes" and suggests to put
  all plugins of the same type in the appropriate directory. However,
  this is out of our control, as the plugins come from other
  applications. What's the best way to approach this problem?