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-privacy.5.scd

@@ -32,6 +32,11 @@ the screen or playing audio.
 	Which privacy modules to monitor. See *MODULES CONFIGURATION* for++
 	more information.
 
+*expand*: ++
+	typeof: bool ++
+	default: false ++
+	Enables this module to consume all left over space dynamically.
+
 # MODULES CONFIGURATION
 
 *type*: ++