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/CMake/code_generators.cmake

@@ -18,6 +18,22 @@ function(stringify_gml source output string_name)
     add_dependencies(all_generated generate_${output_name})
 endfunction()
 
+function(compile_gml source output)
+    set(source ${CMAKE_CURRENT_SOURCE_DIR}/${source})
+    add_custom_command(
+        OUTPUT ${output}
+        COMMAND $<TARGET_FILE:Lagom::GMLCompiler> ${source} > ${output}.tmp
+        COMMAND "${CMAKE_COMMAND}" -E copy_if_different ${output}.tmp ${output}
+        COMMAND "${CMAKE_COMMAND}" -E remove ${output}.tmp
+        VERBATIM
+        DEPENDS Lagom::GMLCompiler
+        MAIN_DEPENDENCY ${source}
+    )
+    get_filename_component(output_name ${output} NAME)
+    add_custom_target(generate_${output_name} DEPENDS ${output})
+    add_dependencies(all_generated generate_${output_name})
+endfunction()
+
 function(compile_ipc source output)
     if (NOT IS_ABSOLUTE ${source})
         set(source ${CMAKE_CURRENT_SOURCE_DIR}/${source})