gtk2 test: group EventMotion tests
gtk2 test: indent
gtk2: add missing methods derived from GdkEventAny to all event classes

TODO: We should fix it in gdk3 too.

GitHub: fix #222

Reported by mtasaka. Thanks!!!
gdk3 test: fix wrong assertion
gdk3: add missing methods derived from GdkEventAny to all event classes

GitHub: #222

Reported by mtasaka. Thanks!!!
gtk3: implement new Gtk::IconView function

`gtk_icon_view_get_cell_rect` is provided in gtk3 3.6 or later.
gtk3: remove trailing spaces