Use UTF-8 instead of narrow

Use wide_to_utf8 and utf8_to_wide instead of wide_to_narrow and narrow_to_wide at almost all places.
Only exceptions: test functions for narrow conversion, and chat, which is done in a separate commit.

src/fontengine.h

@@ -59,7 +59,7 @@ public:
 			unsigned int font_size=FONT_SIZE_UNSPECIFIED,
 			FontMode mode=FM_Unspecified)
 	{
-		return getTextWidth(narrow_to_wide(text));
+		return getTextWidth(utf8_to_wide(text));
 	}
 
 	/** get text width if a text for a specific font */