Rework tagging commands

Unify current commands `tag`, `untag`, and `tags` to
```
$ timew tags add [...] ...
$ timew tags remove [...] ...
$ timew tags list [...]
```
Add a command to add a tag description:
```
$ timew tags describe
```
Add a command to rename a given tag:
```
$timew tags rename
```
Add a command to replace a tag with another for a given ID:
```
$ timew tags replace [...]
```
As usual, when no `ID` is given `@1` is assumed, if there is active time tracking.

Old commands could be restored via (user defined) command aliases, see #144.

Start by tracing the existing `tag`, `untag`, and `tags` command entry points, then review the command-alias context in #144. Done means the unified `tags add`, `remove`, `list`, `describe`, `rename`, and `replace` commands support the stated syntax and the documented default-ID behavior.

Written by the indexing model from the issue text.