Please describe the problem:
I use a laptop with an additional screen that is not always connected. 

So I basically switch between two different X configurations:
* Home: 2 monitors (laptop+additionnal) -> 1 virtual screen with Xinerama
* Out: 1 monitor (laptop)

When home, I have 3 panels:
* 1 at the top of the laptop monitor,
* 1 at the bottom of the laptop monitor with a window list,
* 1 at the bottom of the additionnal monitor with a window list,
so that I have the lists of windows in the panels corresponding to the windows
on that monitor - I was even surprised that this feature existed when I first
tried it :)

The problem is that if I restart with a single monitor, the panel of the
additional screen is moved on the only screen remaining... and I get 2 panels at
the bottom of my screen (and one of these is useless)! And of course if I then
restart with the two monitors, the panel remains on the laptop one and doesn't
move back to the additionnal.




Steps to reproduce:
1. Start X with 2 monitors, using Xinerama
2. Create 2 panels, one on each monitor
3. Restart X with a single monitor
-> The 2 panels are moved on the single monitor
4. Restart X with 2 monitors
-> The 2 panels stay on the last monitor


Actual results:
As stated above, the panels placing behaviour is not convenient when switching
from one configuration to another.

Expected results:
I would expect the panels configuration to be different according to the X
Xinerama configuration:
* 2 monitors -> configuration A (2 panels)
* 1 monitor -> configuration B (1 panel)

Does this happen every time?
Yes

Other information:
I think a good way to manage these kind of situation would be to have different
profiles, based on the Xinerama configuration. This would be really nice for
laptop users.

Another way, simpler to implement but maybe not as convenient, would be not to
create a panel if it is configured to appear on a non existing monitor...