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.

src/gui/widgets/text_box.cpp

@@ -480,7 +480,7 @@ 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 @@ public:
 
 	std::size_t max_input_length;
 
-	std::string hint_text;
+	t_string hint_text;
 	std::string hint_image;
 };