group module: configurable orientation

currently, the orientation of group modules is always the opposite of
the bar. Change it so that:

* the default orientation of the group module is always the opposite of
  its parent, even for nested groups
* the orientation can be overridden in the config
* css ID and class are set for the group element

man/waybar.5.scd.in

@@ -242,6 +242,7 @@ A module group is defined by specifying a module named "group/some-group-name".
 	"modules-right": ["group/hardware", "clock"],
 
 	"group/hardware": {
+		"orientation": "vertical",
 		"modules": [
 			"cpu",
 			"memory",
@@ -253,6 +254,8 @@ A module group is defined by specifying a module named "group/some-group-name".
 }
 ```
 
+Valid options for the (optional) "orientation" property are: "horizontal", "vertical", "inherit", and "orthogonal" (default).
+
 # SUPPORTED MODULES
 
 - *waybar-backlight(5)*