Simplify Workspace::sendClientToDesktop()

This allows changing the type of desk to QVector<VirtualDesktop *>.

Based on the dont_activate flag, Workspace::sendClientToDesktop() will
try to focus the window if it's moved to the current virtual desktop.

In order to implement that, it needs to know whether the window has been
on the current desktop. c->isOnDesktop(desk) is a much sophisticated way
to do that.