Add option to delete template/theme/imageset when deleting style.

When you remove a style via ACP (->Styles, Delete), corresponding row is removed from phpbb_styles, but in three dependent tables (phpbb_styles_template,phpbb_styles_imageset,phpbb_styles_theme) remains rows with a references to a style name and a path to a corresponding data (template/imageset/theme). If another style with the same name is added, this information is not updated either.

Therefore, if later for some reason you decide to install the style with the same name as before, but uploaded to a different path, it will be installed, but the path (in those 3 dependent tables) will not be updated. When using this 'new' style the board will look to the old location. This can lead to very confusing experience, especially when you used to have, say, a 'my_prosilver' style located in 'styles/prosilver_tmp', and then after some tests and tweaks you decide to make a 'stable' branch, uninstall it clearly from the board, rename dir to something like 'styles/prosilver_my_stable', install the path again... whoops, board error, cannot find template data in 'styles/prosilver_tmp'.