Added GtkTreeViewColumn:cell-area construct-only property.

Added GtkTreeViewColumn:cell-area construct-only property.

Allow feeding treeviewcolumn a custom cell-area (or not a custom one,
but allow sharing the cell-area with say, the combo-box area).

This patch also:

 - Fixes signal connections to the area (now they do eventually get
   disconnected at dispose time, they are handled regardless if a
   treeview is set but execute safely, at least there is only one
   connection/disconnection).

 - Fixes refcounting on the cell_area (GtkCellArea is GInitiallyUnowned).

 - Adds a constructor() in order to build the cell-area if one has
   not been provided by the caller before hand at g_object_new()
   construct time.