Before this, vacant tags would show with `hide-vacant` set on initial
startup, because we receive initial tag events from River before we show
the bar. In that case, we won't call `set_visible(false)` on the
respective buttons because they're not shown yet. This registers the
output status listener after we show the bar so we won't miss any
events.
Previously, the only way to select all the module labels was with the
following kind of selector:
```css
.modules-left > widget > label,
.modules-center > widget > label,
.modules-right > widget > label {
/* ... */
}
```
(and a matching block for the `box` containers).
Now, this can be expressed as
```css
label.module, box.module {
/* ... */
}
```
There were two main issues with fmtlib and C++20 mode:
- `fmt::format` defaults to compile-time argument checking and requires
using `fmt::runtime(format_string)` to bypass that.
- `std::format` implementation introduces conflicting declarations and
we have to specify the namespace for all `format`/`format_to` calls.
adds the set-tags and toggle-tags setting so it's possible to have
different tags set vs toggled. This enables the use of e.g. sticky tags
Also clean-up the code a bit.
