This Patch allows the stretching of modules-{left,center,right} as well
add a "expand" flag to AModule. This allows one module to consume the
leftover space.
To allow the left or right modules to fully consume the center, the
changes also include a way to remove the center box (center_)
altogether.
You can configure what key launch the menu with the "menu" element in
the config, the xml file that describes the menu with the "menu-file"
element in the config, and the actions of each buttons with the
"menu-actions" field.
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>
