pager: Add wnck_pager_set_scroll_mode (Fixes #134)

There are now two scroll modes: the default 2d scroll mode is essentially only useful for touchpads, because mice cannot scroll horizontally, so scrolling up/down to get to the next workspace will get users confused. By setting the scroll_mode to 1 users get a simple way of scrolling through workspaces, irrespective of the number of rows the pager sets.