Not able to use backend gtk

I would like to use the backend gtk, because the backend qt conflicts with my QML GUI (see:
https://github.com/barche/QML.jl and https://github.com/barche/QML.jl/issues/5 ).
But I get the following error:

```
julia> using PyCall
julia> pyimport("gtk")
ERROR: PyError (:PyImport_ImportModule)
ImportError('No module named gtk',)

[inlined code] from /home/ufechner/.julia/v0.4/PyCall/src/exception.jl:81
in pyimport at /home/ufechner/.julia/v0.4/PyCall/src/PyCall.jl:285
julia>
```

I tried to install pygtk in different ways. First:

```
import Conda
push!(Conda.CHANNELS, "https://conda.anaconda.org/jochym")
Conda.add("gtk")
```

Then I tried:

```
import Conda
push!(Conda.CHANNELS, "https://conda.anaconda.org/pkgw")
Conda.add("gtk3")
```

Both approaches did not help. I am using Ubuntu 14.04, 64 bit and Julia 0.42.
Any ideas?