Allow overriding hard-coded strings by system3.ini

Some strings ("戻る" and "次のページ" in menu, character names in DPS)
are not included in game files but are hard-coded in the executable.

Now those strings can be overridden by adding [string] section to
system3.ini. For example, system3.ini for English translation of DPS
may look like this:

  [config]
  game  = dps
  title = D.P.S - Dream Program System

  [string]
  back        = Back
  next_page   = Next Page
  dps_custom  = Custom
  dps_linus   = Linus
  dps_katsumi = Katsumi
  dps_yumiko  = Yumiko
  dps_itsumi  = Itsumi
  dps_hitomi  = Hitomi
  dps_mariko  = Mariko

src/sys/nact_sys2.cpp

@@ -490,7 +490,7 @@ void NACT_Sys2::cmd_open_obj(int verb)
 	// 戻るを追加
 	ags->menu_dest_x = 2;
 	ags->menu_dest_y += 2;
-	ags->draw_text(strings.back);
+	ags->draw_text(strings.back.c_str());
 	id[index++] = 0;
 	ags->menu_dest_y += ags->menu_font_size + 2;
 	ags->draw_menu = false;