action: teach MoveTo more sophisticated x/y usage

...including:

1. Negative values (changing the frame of reference to the opposite edge)
2. Percentage of output
3. "center" window within output

Ref:
- https://openbox.org/help/Actions#MoveResizeTo

Fixes: #3611

docs/labwc-actions.5.scd

@@ -77,6 +77,18 @@ Actions are used in menus and keyboard/mouse bindings.
 *<action name="MoveTo" x="" y="" />*
 	Move to position (x, y).
 
+	*x* and *y* take the following types of value:
+	- A positive number like *10*. This will position the window at the
+	  specified distance (in pixels) from the top/left edge of the output
+	  (monitor).
+	- A negative number like *-5* which will similarly position the window
+	  but relative to the bottom/right edge.
+	- The string *center* which will center the window relative to the
+	  output in that dimension
+	- A percentage like *33%* which is interpreted as a relative value of
+	  the output dimensions and will position the window by that amount
+	  from the top/left edge.
+
 *<action name="ResizeTo" width="" height="" />*
 	Resize window.