Track window positions when "select window" has been used

[Yuri D'Elia]

We should update the location of the output when a window has been selected interactively.
Since commit 779976a we can already differentiate between a region and get the XID of the selected window.
As a start, we can update the coordinates when the window is shown.

There is some extra bookkeeping to do, and a CLI parameter to get the XID.