Mod+Wheel Window Resize

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.