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-pulseaudio-slider.5.scd

@@ -27,6 +27,11 @@ The volume can be controlled by dragging the slider across the bar or clicking o
     default: horizontal ++
     The orientation of the slider. Can be either `horizontal` or `vertical`.
 
+*expand*: ++
+	typeof: bool ++
+	default: false ++
+	Enables this module to consume all left over space dynamically.
+
 # EXAMPLES
 
 ```