Normalize header names and include path layout to KF standards

• prepare KF6 KAuthWidgets library, with an interface lib for KF5
• #include <KAuthAction> -> #include <KAuth/Action>, matching C++ names
• drop module generic <KAuth> header, require explicit include per class
• install KAuthCore & KAuthWidgets headers in separate domains, matching the library names and ensuring visibility
• use already KF5/KAuthWidgets module include prefix to match future name and future-compatible imported target
• install backward-compatible headers making use of deprecation macro control flags
• install versiom header in module-named prefix (no change needed here)

Tested with other KF modules using KAuth(Core), builds and runs unchanged and when changed to new names.

Porting away would look like this, getting includes matching the names of the used classes:

• kconfigwidgets!107 (merged)
• kio!763 (merged)
• ktexteditor!288 (merged)
• kcmutils!81 (merged)

Beware: not compatible with existing build dirs & install dirs, needs cleanup, otherwise there will be clashes.

@mlaurent @davidedmundson @vkrause @ahmadsamir