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-clock.5.scd

@@ -96,6 +96,10 @@ $XDG_CONFIG_HOME/waybar/config ++
 :[ array
 :[
 :[ The actions corresponding to the buttons of the menu.
+|[ *expand*:
+:[ bool
+:[ false
+:[ Enables this module to consume all left over space dynamically.
 
 View all valid format options in *strftime(3)* or have a look https://en.cppreference.com/w/cpp/chrono/duration/formatter