HackStudio+LibGUI: Implement matching curly brace highlighting

This works for C++ syntax highlighted text documents by caching the C++
token type in a new "arbitrary data" member of GTextDocumentSpan.

When the cursor is placed immediately before a '{' or immediately after
a '}', we highlight both of these brace buddies by changing their
corresponding spans to have a different background color.

..and spans can also now have a custom background color. :^)

DevTools/HackStudio/main.cpp

@@ -423,7 +423,7 @@ int main(int argc, char** argv)
         remove_current_editor_action->set_enabled(g_all_editor_wrappers.size() > 1);
     };
 
-    open_file("test.frm");
+    open_file("main.cpp");
 
     update_actions();
     return app.exec();
@@ -484,6 +484,7 @@ static void rehighlight()
         span.color = style.color;
         span.font = style.font;
         span.is_skippable = token.m_type == CppToken::Type::Whitespace;
+        span.data = (void*)token.m_type;
         spans.append(span);
     }
     current_editor().document().set_spans(spans);