Keep dock on primary head if KeepDockOnPrimaryHead is YES
This implements a feature request [1] to allow the possibility of
keeping the dock on the primary head on a multi-head system.
In particular, if the new KeepDockOnPrimaryHead option is set to YES,
the dock will either be on the left- or right-hand side of the primary
screen. If it is NO, then we get the current behavior, i.e., the dock
will either be on the left-hand side of the leftmost head or the
right-hand side of the rightmost head.
[1] https://github.com/window-maker/wmaker/issues/24
Closes: #24