Adds several new functions and type for manipulating "simple windows".
There are several ways to draw windows in X.U.XUtils and other places
already, but they are all quite manual. Most of the time one does not
want to think about dimensions much and assumes that the code is smart
enough to "figure it out"; this is an attempt to do exactly that.
There is a less-managed version `showSimpleWindow`, which just creates
and shows the windows, as well as a wrapper-like `withSimpleWindow` that
also destroys the window once some action finishes executing.
With these functions it should be possible to refactor some contrib
modules that currently draw windows manually, like X.U.EasyMotion.