simplify + improve data import UI; data loaders; docu

* RealDataSelectorWidget:
   ** integrate actions
   ** integrate tree view
   ** remove indirections
   ** selection view redesigned (tree, icons, headlines)
   ** dedicated model for tree view
   ** item selection handling
* add icons
* refactor import process
* implement legacy data loader
* improve QRE loader
* improve error reporting
* better file filters for import dialog
* remember file filter of import dialog