Add scratchpad.

It still flickers when changing the workspace, might want to search for
a more performant solution (probably by not attaching the scratchpad to
a workspace, but treating it as a "special" client).
Update config and README.
Handle scratchpad by monitor, not by workspace.

This prevents flickering and allows finer controll over the scratchpad
windows.

... and makes the WM hang, occasionally (will get tracked down).
Integrate scratchpad client handling more deeply.

Especially unmapping the scratchpad is quite delicate, but it seems to
be quite stable.
Adapt scratchpad for multi-monitor.

There is an issue with the focus when positioning the mouse on the
monitor that's got the scratchpad on it, then Alt+M to another monitor
and toggling the scratchpad from there; the focus stays on the old
monitor.

But I guess it won't happen very often ^^
Merge branch 'scratchpad'.

Implements a window that can quickly be displayed and dismissed.