Split out Activities related code from Workspace

All activities related code moves into new singleton class Activities.
This class gets only included into the build if the build option is
enabled which means there are less ifdefs all over the code and it also
handles better the moc doesn't like ifdef case.

The class holds the list of open and all activites, the current and the
previous activity and the KActivities::Controller. It also emits the
signals for any activities related changes.

Workspace still contains some activities related code. That is the
adjustment on change of current activity. Nevertheless the code looks
much cleaner now and does not contain the confusing naming conflict with
takeActivity() which existed before.

In all the places where Activities got used the code got adjusted and
quite often the ifdef got added with a fallback for the disabled case.