Various fixes needed to get `tests.jl` running

Finally starting to play with this. At least on my machine (see below for library versions), these seem necessary. Still, since I'm new to Gtk it seemed better to submit as a PR.

Even with these fixes, when I run tests.jl I get no errors but I do get this warning:
```
julia> include("tests.jl")
WARNING: set_position(w, 10, 10)

(julia:4532): Gtk-WARNING **: /build/buildd/gtk+3.0-3.4.2/./gtk/gtkcontainer.c:1128: container class `GtkBox' has no child property named `page_type'

(julia:4532): Gtk-WARNING **: /build/buildd/gtk+3.0-3.4.2/./gtk/gtkcontainer.c:1128: container class `GtkBox' has no child property named `page_type'
```
I investigated a little bit, but there's no `set_position` function, so I'm confused about what's happening here.

Details on my library versions:
```
tim@diva:/usr/lib/x86_64-linux-gnu$ ls libgtk*
libgtk-3.so.0        libgtkmm-2.4.so.1      libgtk-x11-2.0.a   libgtk-x11-2.0.so.0
libgtk-3.so.0.400.2  libgtkmm-2.4.so.1.1.0  libgtk-x11-2.0.so  libgtk-x11-2.0.so.0.2400.10

libgtk2.0-0:
gtk-query-immodules-2.0  gtk-update-icon-cache

libgtk-3-0:
gtk-query-immodules-3.0  gtk-update-icon-cache-3.0
```
Gtk reports that it is using version 3.