Remove Unused Usings and the Includes Kept Only for Them

151 using-directives, using-declarations, type aliases and namespace
aliases in source and test files that nothing refers to: the name is
never used, it duplicates a using already in scope, or the code sits
inside the namespace it names. Each one was removed on its own and the
file still compiled, both as it is and with every header-level using
taken away, so none of them was only redundant because a header leaks
the same name.

With the using gone, 28 #include lines and one forward declaration had
no other reference left in their file (boost/optional.hpp without any
optional, property_tree headers without any ptree) and go with it.

No header is touched.

src/slic3r/GUI/ConfigWizard.cpp

@@ -95,7 +95,6 @@ namespace GUI {
 
 
 using Config::Snapshot;
-using Config::SnapshotDB;
 
 
 // Configuration data structures extensions needed for the wizard