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.cpp

@@ -25,7 +25,7 @@ NACT::NACT(int sys_ver, uint32 crc32_a, uint32 crc32_b, const Config& config)
 	  crc32_b(crc32_b),
 	  config(config),
 	  encoding(Encoding::create(get_encoding_name())),
-	  strings(get_language(), encoding.get())
+	  strings(config.get_strings(encoding.get(), get_language() == ENGLISH))
 {
 	platform_initialize();