AK: Add String::number() for creating a String from a number.

Instead of manually doing String::format("%d"/"%u") everywhere, let's have
a String API for this. It's just a wrapper around format() for now, but it
could be made more efficient in the future.

LibGUI/GVariant.cpp

@@ -289,7 +289,7 @@ String GVariant::to_string() const
     case Type::Bool:
         return as_bool() ? "true" : "false";
     case Type::Int:
-        return String::format("%d", as_int());
+        return String::number(as_int());
     case Type::Float:
         return String::format("%f", (double)as_float());
     case Type::String: