Remove obsolete tooltip database (xml & related class). The tooltip database was only used in one spot. To simplify the code, these tooltips can be defined
directly where they are used.

Some of the tooltips are also present in item classes (and therefore duplicate). This may be worth more refactoring in the future.

* 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

Redmine: #1618

Not functional for now

New IntensityDataWidget as a replacement of heavy IntensityDataPlotWidget. Corresponding icons are changed to their svg versions.

Ellipse and Rectangle can be drawn in every direction, Changed polygon icon, naming of items are correctly displayed and refractored code