Changing control font doesn't work on Linux
Hi,

I have a simple app with a textbox and a button in a container.
On Windows, setting the font for either textbox or button works fine, but on Linux nothing happens. The font family and the font size changes have no effect. The code is:
```nim
var textbox = newTextbox("00:00:00")
textbox.fontSize = 50
textbox.fontFamily = "Courier"

var button = newButton("START")
button.width = 100
button.height = 40
button.fontSize = 30
button.fontFamily = "Courier"
```
I am testing on a `Raspberry Pi 3 (Raspbian)` with `Nim Compiler Version 0.18.1 [Linux: arm] Compiled at 2018-05-22`
