If a window is on several desktops, AbstractClient::desktop() will
return the id of the last desktop.

For example, if a window is on virtual desktops A and B, the desktop()
function will return the id of desktop B. This can be the culprit for
bugs such as window snapping not working as expected when moving a
window on virtual desktop A, e.g.

- moved window is on desktops A, and C. desktop() returns the id of C
- snap candidate (l) is on desktops A, and B. desktop() returns the id
  of B

Even though the snap candidate window and the moved window are both on
the same desktop (A), the moved window won't be snapped because the
desktop() function returns garbage values.

To fix that, the workspace needs to check whether the window is on the
current desktop.

For what it's worth, that's also how the workspace handles windows being
on multiple activities.