LibGUI: Add GMLFormatter

This will be useful for both the Playground app as well as for a
standalone CLI tool (à la clang-format). It cannot handle comments yet
(and will drop them from the formatted output), but other than that it
produces valid GML that matches the formatting we have so far! :^)

Libraries/LibGUI/CMakeLists.txt

@@ -36,6 +36,7 @@ set(SOURCES
     FontPicker.cpp
     FontPickerDialogGML.h
     Frame.cpp
+    GMLFormatter.cpp
     GMLLexer.cpp
     GMLParser.cpp
     GMLSyntaxHighlighter.cpp