Added "edit_only" argument to gtk_cell_area_activate()

Added "edit_only" argument to gtk_cell_area_activate()

This argument allows the caller to specify that only an editable
cell should start editing but an activatable cell should not toggle
it's state, this is important for public apis like
gtk_tree_view_set_cursor_on_cell() which are only intended to
programatically bring attention to the editing of a specific
row or cell but not actually change any data.

GtkTreeView & CellAreaScaffold updated for the last minute api change.