Add gui to edit FeatureModels

[Sinerum]

Adds a gui tool to view featuremodels including the sources for features and edit them.

[LuAbelt]

Apart from what Flo and Lauritz already mentioned, some small things I found while using the editor (Mostly minor inconveniences):

• Newly added features don't appear in the TreeView
• When navigating through the TreeView with the arrow buttons the information panel does not update
• When adding new features it can happen that these are created outside of the navigatable area in the GraphView
• Clicking AddSource for a feature without source file creates an "empty" (Displayed as [:1:1-:1-1]) location
• The current filter for C files in the add source file dialog might be too restrictive for some projects (E.g. Dune uses .hh and .cc, other extensions that come to mind currently not included are .hpp, .cxx, .cpp). I think the most flexible way to handle this is to just add an additional filter to the dialog that displays all files. Then you also don't run into any issues when some projects might use non-standard fiel endings
• It would be nice to also be able to create new feature models from scratch
• When saving it would be more convenient if it just saves the current feature model to it's known location without opening the file dialog everytime. The current behavior could be retained with a "Save as" functionality

[LuAbelt]

Another thing I just noted that is not only an inconvenience:

The QTextEdit used to display the source files currently wraps lines that exceed the widgets width. Since the line/column in the textedit then do not represent the actual position in the source files, this causes wrong sources to be added to the FM.
You probably change the wrap mode to QTextEdit::LineWrapMode::NoWrap.