Handle transformed attr name plus refactoring

This PR uses https://github.com/gem/qt-experiments/pull/40 (which solves the problem of wrong calculation of the indicator averages)

There are a number of renaming, reordering and improvements to the plugin's workflow and user interface, that were necessary to provide an almost full demonstration of the tool using loss and socioeconomic data collected in Portugal. 
Apart from the intensive refactoring, the biggest improvements are:
* the result of a field transformation get saved into a new field which name is decided by the user through an additional textfield in the transformation dialog; if the name is not specified, the new field gets by default the name of the transformation method. In case the name of the attribute is already taken by another field in the layer, before this PR there was a problem in the creation of the new field, which now has been solved
* the dialog to build the weighting tree, that allowed to select all the field in the active layer, now only displays the numeric fields (for which the calculations are meaningful)
* small bugfix in the dialog for field transformations. The transformation variant combobox gets re-enabled when it's populated as a consequence of the selection of the transformation method (otherwise, selecting a method that doesn't have any variants, the variant combobox would be permanently disabled)