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/debugger/editor_debugger_inspector.h

@@ -61,7 +61,7 @@ public:
 
 	void update() { _change_notify(); }
 
-	EditorDebuggerRemoteObject(){};
+	EditorDebuggerRemoteObject() {}
 };
 
 class EditorDebuggerInspector : public EditorInspector {

editor/debugger/editor_debugger_node.h

@@ -76,7 +76,7 @@ private:
 			return line < p_b.line;
 		}
 
-		Breakpoint(){};
+		Breakpoint() {}
 
 		Breakpoint(const String &p_source, int p_line) {
 			line = p_line;