title: Gtk.jl getting started example button issue
body: <p>I want to make a simple GUI application using Julia and Gtk, however when trying the Getting started example on Gtk.jl the button does not show up.</p>
  
  <p>I am just getting started using both Julia and Gtk, and want to do a simple GUI application. However, following the very first Gtk.jl getting started example the window, but not the button, shows. I am on macOS Mojave.</p>
  
  <p>At first I ran into a locale warning when importing the package:</p>
  
  <pre><code>"Gtk-WARNING **: 12:50:44.014: Locale not supported by C library. Using the fallback 'C' locale."
  </code></pre>
  
  <p>, however ignoring or fixing it using</p>
  
  <pre><code>export LC_ALL="en_US"
  </code></pre>
  
  <p>have made no difference. The example code (and installation) goes as follows (and I run it line-by-line in the REPL):</p>
  
  <pre><code>#One time installation
  Pkg.add("Gtk")
  
  #Getting started example
  using Gtk
  
  win = GtkWindow("My First Gtk.jl Program", 400, 200)
  
  b = GtkButton("Click Me")
  push!(win,b)
  
  showall(win)
  </code></pre>
  
  <p>Which produce the expected window, but not the expected button.</p>