xfce4-panel: multi-monitor profile support

[Michael Wyraz]

Hello,

I hope this is the right place for discussing this. I'm using xfce4 for 
years and I really like it. I has also good multi-monitor support, the 
detection feature with auto-applied profiles is perfect.

But there's one thing that bugs me a lot: the missing support for 
changing monitor topography in xfce-4panel. For example on my desk I 
have 2 large displays connected to the docking station. On each I have 
an xfce4-panel with window buttons for that display. On one I also have 
the notification area. When I unplug the laptop, I have only one screen. 
So the 2nd panel moves to that one screen, hiding the other panel 
(including the notification area).

My current workaround is a little script bound to a hotkey that toggles 
the width from the 2nd panel between 1% and 100% (0% or "hidden" is not 
supported). There are other solutions around, all have their drawbacks 
(e.g. one wrote a tool that saves/restores panel configurations - 
drawback is that on every change to the panel, the stored configs needs 
to be updated).

I thought about how this could easily be solved:

- 1st allow a panel to be set to "hidden" or "deactivated", so that the 
panel is not displayed but it's configuration is kept

- add an option to the panel to show it only if there are N 
(configurable number, default 1) or more monitors connected

- alternatively add an option to the panel to show it only if a certain 
screen profile is active

- if screen topography changes, trigger a panel update.

What do you think about this?

Unfortunately I don't have good C programming skills nor xfce4 knowledge 
to do this via PR but I'd be happy to help e.g. with testing, docs, 
translation (german) and feedback.

Best regards,

Michael.