gtk2: set_cursor is a property.

see: https://developer.gnome.org/gdk2/stable/gdk2-Windows.html
gtk2: remove trailing spaces
gtk2 test: add cursor accessor tests.
gtk2: In Gtk::Widget, window is `Read` property since gtk+2 2.14.0.

see: https://developer.gnome.org/gtk2/2.24/GtkWidget.html#GtkWidget--window
gtk2: remove trailing spaces