ref #19

- count how often tags are used
- improved structure for ACP settings
- acp option for tagcloud to be displayed on index or not (not used, yet)
- acp option to limit count of tags shown in tagcloud (not used, yet)
further improvements of tag cloud ref (ref #19)

- cleaned css code
- dynamic tag-size in tag cloud depending on its usage count
- acp option to en/disable display of usage count of tags in tag cloud