Remove desktop argument in AbstractClient::electricBorderMaximizeGeometry()

With the new virtual desktop model, we have an issue where the old code that uses desktop() needs to be ported to desktops().

However, using no desktop() is better as we don't need to deal with cases where a window can be on several desktops, which can be annoying sometimes.

This change removes the desktop arg in electricBorderMaximizeGeometry() and ports it to a Workspace::clientArea() overload that requires no desktop.

Under the hood, Workspace::clientArea() still uses desktop(), but it could also use a different strategy to compute the client area if the window is on several virtual desktops, e.g. intersect client area on every virtual desktop.