Meta+Userland: Allow generating C++ initializer code from GML

This does the exact same thing as the runtime initializer,
except it is faster and can catch some errors much earlier.

The code generator includes these important features:
- Automatic include generation where necessary
- Special-casing for TabWidget and ScrollableContainerWidget
- No use of DeprecatedString where possible

Meta/Lagom/CMakeLists.txt

@@ -370,6 +370,17 @@ install(TARGETS LibTimeZone EXPORT LagomTargets)
 # This is used by the BindingsGenerator so needs to always be built.
 add_serenity_subdirectory(Userland/Libraries/LibIDL)
 
+# LibGUI - only GML
+# This is used by the GML compiler and therefore always needed.
+set(LIBGUI_GML_SOURCES
+    GML/Lexer.cpp
+    GML/Parser.cpp
+)
+list(TRANSFORM LIBGUI_GML_SOURCES PREPEND "${SERENITY_PROJECT_ROOT}/Userland/Libraries/LibGUI/")
+lagom_lib(LibGUI_GML gui_gml
+    SOURCES ${LIBGUI_GML_SOURCES}
+)
+
 # Manually install AK headers
 install(
     DIRECTORY "${SERENITY_PROJECT_ROOT}/AK"