Add "Center" window placement strategy

Center strategy: try to put window at the center of the usable area. If
window would overlap with existing windows, fall back to "Auto"
placement strategy.

It's very useful for fresh workplaces.

src/WindowMaker.h

@@ -153,6 +153,7 @@ typedef enum {
 #define WPM_SMART	2
 #define WPM_RANDOM	3
 #define WPM_AUTO        4
+#define WPM_CENTER      5
 
 /* text justification */
 #define WTJ_CENTER	0