Turn TEMPLATE_BITFIELD into an instance variable in acp_styles.php

acp_styles.php has this code:

		define('TEMPLATE_BITFIELD', $bitfield->get_base64());

This prevents more than one style to be refreshed in a single script invocation if the most obvious approach to doing so is taken (which is multiple instantiations of acp_styles class).

I do not see any reason why this should be a global define, therefore the proposal is to make it an instance variable instead.