Mod+Wheel Window Resize

This patch adds the ability to resize windows with the mouse wheel
while holding the Mod key. This currently ignores wWindowConstrainSize
until I can figure out a way to repeatably resize windows with
fixed size increments (like xterm) using this method.

This also adds a slider to WPrefs to choose the increment with which
the wheel will resize a window.

src/WindowMaker.h

@@ -466,6 +466,7 @@ typedef struct WPreferences {
     signed char shade_speed;
 
     int edge_resistance;
+    int resize_increment;
     char attract;
 
     unsigned int workspace_border_size; /* Size in pixels of the workspace border */