Tidy Up Constructors and Destructors

This commit does not add or remove c’tors or d’tors, so it is *not* ABI-breaking.

If a c’tor/d’tor does nothing else than the default behavior, this commit replaces it with “= default”.

If an initializer list entry does nothing else than the default behavior, this commit removes it. First and foremost, remove default c’tor calls of base classes (always called by the compiler if no other base c’tor is explicitly called) and c’tor calls of members with complex types (e.g. “std::vector”).

In a few instances, user-defined copy c’tors / move c’tors / assignment operators / move assignment operators were replaced with “= default”, too. I only did this if I had a clear understanding of what’s going on.

code/AssetLib/X/XFileImporter.cpp

@@ -75,9 +75,7 @@ static const aiImporterDesc desc = {
 
 // ------------------------------------------------------------------------------------------------
 // Constructor to be privately used by Importer
-XFileImporter::XFileImporter() : mBuffer() {
-    // empty
-}
+XFileImporter::XFileImporter() = default;
 
 // ------------------------------------------------------------------------------------------------
 // Returns whether the class can handle the format of the given file.