gui2/text_box: Store hint text as a t_string

Storing it in a regular string results in issues with the game getting
stuck with the translation for the locale that was set during GUI2
initialization and not reflecting changes when switching languages later
on.

(cherry-picked from commit eacbc5f)

src/gui/widgets/text_box.cpp

@@ -481,7 +481,7 @@ builder_text_box::builder_text_box(const config& cfg)
 	: builder_styled_widget(cfg)
 	, history(cfg["history"])
 	, max_input_length(cfg["max_input_length"])
-	, hint_text(cfg["hint_text"])
+	, hint_text(cfg["hint_text"].t_str())
 	, hint_image(cfg["hint_image"])
 {
 }

src/gui/widgets/text_box.hpp

@@ -324,7 +324,7 @@ struct builder_text_box : public builder_styled_widget
 
 	std::size_t max_input_length;
 
-	std::string hint_text;
+	t_string hint_text;
 	std::string hint_image;
 };