Added directory selection dialog and improved preference UI

[ghost]

• The user can now select in the preferences if he always wants to use the same path defined there or if he should be asked
  • Minor change in the plugin interface, the path defined in the command is now also handed over, to remove unnecessary dialog calls
• Clearer seperation of plugin and general preference UI:
  • Plugins now contribute a complete UI page displayed in a new tab of the tools preference page
  • Also improve UI spacing and layouting