Is it possible to create gtk object/widget which will appear at glade?
Glade allows for "custom" widgets--I believe by using a "placeholder" which will then attempt to load your widget by name. I've never done that though, you'd have to check with the glade-users list.
I want to start with a timer widget.
GLib already provides this functionality in g_timeout_add and the like.
Is it also possible to create non graphics widgets?
For example timer need only property interval and event for each interval.
You could just as easily derive your custom object from GtkObject or GObject, although then we wouldn't call it a "widget" but just an object. For example, GError and GString are some objects.
Is it possible to make those widgets in Python?
I dont' like this idea as I don't think people using C would be able to use your widget, nor would language bindings be created for it in any conventional sense, however, it can probably be done.
Check out: http://www.learningpython.com/2006/07/25/writing-a-custom-widget-using-pygtk/