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 {
/* ... */
}
```
The fmt::format() function looks for the "{arg}" named argument
in the given "format" string which does not exist. It will fail
if the string contains any {...} curled-brace substring.
Consequently, any "on-click*" option's command line containing for
instance substring like "${var}" or "awk '{...}'" will crash the program.
Signed-off-by: Thomas Venriès <thomas.venries@gmail.com>
