Move panopoly_page's hook_menu_block_tree_alter() to panopoly_widgets

Currently, panopoly_pages has a hook_menu_block_tree_alter() that corrects for some weirdness when using menu_block in the IPE.

However, panopoly_pages is a module that is frequently omitted in child distributions AND the menu_block dependency actually comes from panopoly_widgets. We'd really like anyone who's using panopoly_widgets to get that fix!