Port member initialization from constructor to declaration (C++11)

Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.

Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.

editor/editor_node.cpp

@@ -42,7 +42,6 @@
 #include "core/os/file_access.h"
 #include "core/os/keyboard.h"
 #include "core/os/os.h"
-#include "core/path_remap.h"
 #include "core/print_string.h"
 #include "core/project_settings.h"
 #include "core/translation.h"