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
gtk3: add test for Gtk::IconView#get_cell_rect
gtk3 test: add only_gtk_version

Sorry for forgetting to add gtk version check....
gtk3 test: ensure collecting unused Gtk::TreePath
gtk3: remove "rect" from arguments

Because it is an parameter. Ruby/GTK3 should not require
Gdk::Rectangle for users.
gtk3: implement Gtk::HeaderBar

GtkHeaderBar is provided in gtk+3 3.10 or later. see: https://developer.gnome.org/gtk3/3.10/GtkHeaderBar.html

Reported by dark-yux in issue #206.
Thanks!