Icon-grid: Ask the user before re-arranging the icon-grid

Followup of #47 (closed)

On some events (e.g. a monitor is unplugged, the screen resolution is changed, ot the panel size is increased), it happens that icons fall out of the visible scren and cannot be drawn any more at their old position.

Currently such icons will be automatically re-located to the first free place which is found in the icon-grid.

However in many cases that is not desired. (E.g. when just playing with the screen resolution, temporary removing one monitor, or dockable laptop).

While in other cases (E.g. permanbent removal of a screen) it makes sense to re-arrange.

In addition some people (including me) try to arrange related desktop items in clusters. The current automatic re-arranging can quickly destroy such clusters by accident.

Since the use-case that icons cannot be drawn any more happens not that ofter, I think it should be fine to ask the user what to do, instead of doing it automatically. E.g. via notification, like done for xfce4-power-manager.

If the icons can be drawn again (e.g. screen resolution changed back to good values), the notification could be closed.

(If such a notification would be too annoying, a "dont ask me again" checkbox possibly would be an option ... though probably a full dialog would be required)

If no blockers are found and you would be fine with it, I could provide a MR for this after xfce 4.18 is out.