LibGUI: Remove GML prefix in favor of proper namespace

Prefixes are very much a C thing which we don't need in C++. This commit
moves all GML-related classes in LibGUI into the GUI::GML namespace, a
change somewhat overdue.

Userland/Libraries/LibGUI/CMakeLists.txt

@@ -46,11 +46,11 @@ set(SOURCES
     GitCommitLexer.cpp
     GitCommitSyntaxHighlighter.cpp
     GlyphMapWidget.cpp
-    GMLAutocompleteProvider.cpp
-    GMLFormatter.cpp
-    GMLLexer.cpp
-    GMLParser.cpp
-    GMLSyntaxHighlighter.cpp
+    GML/AutocompleteProvider.cpp
+    GML/Formatter.cpp
+    GML/Lexer.cpp
+    GML/Parser.cpp
+    GML/SyntaxHighlighter.cpp
     GroupBox.cpp
     HeaderView.cpp
     INILexer.cpp