RFC: Add SafeDesktopArea class

This new class allows to know the current "safe desktop area". This is the area of the desktop that will never be ocluded by elements other than a window. This is useful for elements that are placed in the desktop, like desktop icons, when there are other elements that can cover the desktop. An example is dash-to-dock or dash-to-panel with the autohide option enabled: in that case, the dock/panel will hide when it is covered by a window, but will appear when no window is in its place. This means that the zone occupied by the dock/panel will never be visible by the user. Also, since the dock/panel hides when a window covers that part, the working area (which is the value used to determine the maximized size for the windows) is not affected by it, so it can't be used for this purpose.

With this class, these dynamic elements can register the zone occupied by them, thus allowing desktop icons or other extensions (like animated widgets) to avoid putting elements in that zone, where the user wouldn't be able to see or manipulate them.

This is a RFC because it is still a proposal. I accept suggestions.

Related bugs: World/ShellExtensions/desktop-icons#209 and World/ShellExtensions/desktop-icons#136