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.pro

@@ -88,6 +88,10 @@ unix {
 		PKGCONFIG += fluidsynth
 	}
 
+	INI_ENCODING {
+		PKGCONFIG += libguess
+	}
+
 	# Deal with boost paths...
 	isEmpty(BOOST_I) {
 		BOOST_I = $$(BOOST_I)
@@ -248,6 +252,10 @@ MIDI {
 	DEFINES += MIDI
 }
 
+INI_ENCODING {
+	DEFINES += INI_ENCODING
+}
+
 defineReplace(xxdOutput) {
 	return($$basename(1).xxd)
 }