Moving tabs between workspaces

When there are multiple workspaces (s.a. in Ubuntu), if you close one of the "middle" workspaces by closing all the windows, other workspaces shift to the empty one, and the last workspace becomes the empty one. If there are multiple empty workspaces in the end, they all collapse into one empty workspace.

However, if you have guake running, after shifiting the terminal of the shifted workspace does not shift with the rest of the windows. I don't think this is a bug -- I don't see a way of preserving the guake tabs while shifting new ones in. However, it would be nice to give an option to the user to manually move a tab from one workspace to another. That way, if there are any tabs open that are related to a specific workspace, the user can manually migrate them once the workspaces shift.

```
$> guake --support
$ guake --support

Guake Version: 3.7.0

Vte Version: 0.52.2

Vte Runtime Version: 0.52.2

--------------------------------------------------
GTK+ Version: 3.22.30

GDK Backend: GdkX11.X11Display

--------------------------------------------------
Desktop Session: ubuntu

--------------------------------------------------
Display: :0

RGBA visual: True

Composited: True

* Monitor: 0 - HDMI-0
* Geometry: 3440 x 1440 at 0, 0
* Size: 800 x 335 mm²
* Primary: True
* Refresh rate: 59.97 Hz
* Subpixel layout: unknown
```