Clean up `region` class

[cschreib]

This PR does a bit of refactoring to remove unused or unnecessary members of region:

• rename region::is_special() into region::is_manually_inherited()
• rename region::border_list_ into region::borders_
• rename region::get_object_type() into region::get_region_type
• removed region::id_
• removed region::get_lua_name() and region::lua_name_
• removed useless overrides of show/hide/is_visible
• removed frame::get_frame_type() and Lua is_frame_type
• removed region::type_, using static list instead
• removed region::lua_members_, using Lua instead
• removed region::previous_anchor_parent_list_

Other changes:

• Fixed text not using atlas for icons; will improve performance
• Fixed renderer::get_batch_count() not implemented, and added renderer::reset_batch_count()
• Added renderer::get_vertex_count()
• Added vertex_cache::get_vertex_count()
• Renamed any function of the form get_num_[...]() into get_[...]_count()
• Renamed region::[get/set/modify]_point to region::[get/set/modify]_anchor
• Renamed region::clear_all_points to region::clear_all_anchors
• Renamed region::set_all_points to region::set_all_anchors