core: Add separate Debug class

Replaces 'debugMessage' by specialized debug function 'Debug::log'.
This function takes the same arguments as the previous 'debugMessage'.

A separate Debug class allows for further optimization and separation
of concern.

lib/Debug.php

@@ -0,0 +1,19 @@
+<?php
+
+class Debug {
+	public static function log($text) {
+		if(!DEBUG) {
+			return;
+		}
+
+		$backtrace = debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, 3);
+		$calling = $backtrace[2];
+		$message = $calling['file'] . ':'
+			. $calling['line'] . ' class '
+			. (isset($calling['class']) ? $calling['class'] : '<no-class>') . '->'
+			. $calling['function'] . ' - '
+			. $text;
+
+		error_log($message);
+	}
+}