Shift to next/previous tag, with skipping occupied/unoccupied variants.
This patch is an alternative to `shiftview` patch with more controls:
- `shifttag`: shift to next/previous tags
- `shifttag_occupied`: skipping unoccupied tags
- `shifttag_unoccupied`: skipping occupied tags
`shifttag-bar` is a variant to be applied on top of the bar patch.
