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/guiPasswordChange.cpp

@@ -203,8 +203,8 @@ bool GUIPasswordChange::acceptInput()
 				e->setVisible(true);
 			return false;
 		}
-		m_client->sendChangePassword(wide_to_narrow(oldpass),
-			wide_to_narrow(newpass));
+		m_client->sendChangePassword(wide_to_utf8(oldpass),
+			wide_to_utf8(newpass));
 		return true;
 }