The new class FocusChain manages two different kind of focus chains.
First of all there is a most recently used focus chain which is primarily
used for TabBox.
Then there is one focus chain per virtual desktop. These chains are used
to determine which Client needs to be activated when e.g. switching to a
virtual desktop.
The individual chains are implemented as a simple QList of Client* with
the most recently used Client as the last element. That way one can see
it as a LIFO like structure.
The desktop focus chains are internally represented as a hash with the id
of the virtual desktop as the key and a list as described as the value.
FocusChain is a singleton which provides some methods to manipulate the
chains and to get a specific Client for a task (e.g. TabBox).
While splitting out the code some unused code inside TabBox got removed
as well as some activities related code (windows cannot be moved while
switching activities).
REVIEW: 107494
