Config: Convert game title to valid UTF-8 (if it isn't already)

Removes the need to manually convert the Game.ini to UTF-8 every
time with eg. Japanese games. Also, setting the window title on
OSX with invalid UTF-8 crashes.

This functionality and the dependency on libiconv and libguess
are optional and can be enabled with `CONFIG+=INI_ENCODING`.
If turned off and invalid UTF-8 is encountered, the game title
is treated as being empty (ie. the folder name is used instead).

mkxp.conf.sample

@@ -163,3 +163,12 @@
 # Activate "reverb" effect for midi playback
 #
 # midi.reverb=false
+
+
+# Give a hint on which language the game title as
+# specified in the Game.ini is, useful if the encoding
+# is being falsely detected. Relevant only if mkxp was
+# built with automatic encoding conversion (INI_ENCODING).
+# (default: none)
+#
+# titleLanguage=japanese