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.

core/io/logger.h

@@ -55,7 +55,7 @@ public:
 	void logf(const char *p_format, ...) _PRINTF_FORMAT_ATTRIBUTE_2_3;
 	void logf_error(const char *p_format, ...) _PRINTF_FORMAT_ATTRIBUTE_2_3;
 
-	virtual ~Logger();
+	virtual ~Logger() {}
 };
 
 /**
@@ -65,7 +65,7 @@ class StdLogger : public Logger {
 
 public:
 	virtual void logv(const char *p_format, va_list p_list, bool p_err) _PRINTF_FORMAT_ATTRIBUTE_2_0;
-	virtual ~StdLogger();
+	virtual ~StdLogger() {}
 };
 
 /**
@@ -78,7 +78,7 @@ class RotatedFileLogger : public Logger {
 	String base_path;
 	int max_files;
 
-	FileAccess *file;
+	FileAccess *file = nullptr;
 
 	void rotate_file_without_closing();
 	void close_file();