Add "Increase/Decrease window height" actions

[jinliu]

The "Stacking offset Y" config is reused as the step size.

[peterfajdiga]

Thanks for the PR.

I have a problem with this approach, though. When you're changing the height of a window, you're also changing the height of another window (its upper or lower neighbor). But with this approach you have no control over which of the neighbors you're affecting.

I'd prefer to add actions like window-height-increase-up and window-height-increase-down. Both of them increase window height, but the first one grows the window upwards (shrinks the upper neighbor) and the second one grows it downwards (shrinks the lower neighbor).

For decreasing the height of a window, I wouldn't add new actions, to avoid too many actions. Instead, the user can switch focus to a neighbor window and grow that one to achieve the same effect.