Add stretching of modules and modules-center toggling

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.

man/waybar-mpris.5.scd

@@ -148,6 +148,11 @@ The *mpris* module displays currently playing media via libplayerctl.
 	typeof: map[string]string ++
 	Allows setting _{status_icon}_ based on player status (playing, paused, stopped).
 
+*expand*: ++
+	typeof: bool ++
+	default: false ++
+	Enables this module to consume all left over space dynamically.
+
 
 # FORMAT REPLACEMENTS