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?

Start with the failing PyCall entry point in PyCall.jl:285 and the exception handling referenced at PyCall/src/exception.jl:81, then inspect how the GTK backend is selected. Reproduce the pyimport("gtk") failure using the listed Conda installation attempts and the Ubuntu 14.04 setup. Done means the GTK backend can be used without the QML GUI conflict and the reported import failure is resolved.

Scritto dal modello di indicizzazione a partire dal testo della issue.