Add option to close windows with mouse middle click on button in tasklist

From http://git.mate-desktop.org/libmatewnck/commit/?id=6d3058d

https://bugzilla.gnome.org/show_bug.cgi?id=704472
TestTaskList: enable middle-click for closing
TaskList: get rid of deprecated code
Selector: get rid of deprecated code
WnckTaskList: Add API to define task list orientation

This allows to build vertical task lists.
The patch comes has been originally made by Jean-Luc Porquet while
Josselin Mouette has been included it in debian (http://is.gd/nDwsfY)

The patch has been cleaned to make the indentation to match the
libwnck rules, and properly commented.
WnckTasklist: no need to initialize priv values to FALSE, NULL or 0
WnckTask: no need to initialize priv values to NULL, FALSE or 0
TestTaskList: add option to draw the test in vertical mode
WnckTaskList: correctly set requisition parameters for Vertical Mode
libwnck: remove dependency on glib-genmarshal, use generic marshaller
WnckWindow: remove unneeded priv parameters initialization to 0
WnckScreen: remove unneeded priv parameters initialization to 0
WnckWorkspace: remove unneeded priv parameters initialization to 0
WnckApplication: remove unneeded priv parameters initialization to 0
WnckClassGroup: remove unneeded priv parameters initialization to 0
WnckPager: remove unneeded priv parameters initialization to 0
WnckSelector: remove unneeded priv parameters initialization to 0
WnckPager: use GtkStypeContext do draw the window, avoid deprecated code
WnckPager: some code cleanup
WnckPager: remove the deprecated usage of GtkSymbolicColor for good

The get_dark_color_for_state function is now replaced by the function
draw_dark_rectangle that draws a rectangle using darker forground color.