Most recently used virtual desktop chain is only used in the context of
TabBox and therefore moved into this namespace. KWin uses one desktop
chain for each activity. This is mapped by having multiple DesktopChains.
In addition there is a DesktopChainManager which contains all those
chains which are identified by a QString.
The manager gets connected to the signals emitted by VirtualDesktopManager
for changes in virtual desktops and to signals related to Activities
emitted by Workspace. This means the manager is rather generic as it does
not depend on any other components.
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