Unwanted padding depending on order of widget addition
When I add the main container to the window at the very beginning (after creating the container), the interface is displayed correctly.
```nim
import nigui

app.init()
let window = newWindow()

let main_container = newLayoutContainer(Layout_Vertical)
window.add(main_container)

let test_checkbox = newCheckbox(
    "Test checkbox")

let test_textarea = newTextArea()
test_textarea.editable = false

let progressBar = newProgressBar()

main_container.add(test_checkbox)
main_container.add(test_textarea)
main_container.add(progressBar)

window.show()

app.run()
```
![Screenshot_2](https://user-images.githubusercontent.com/43933400/104551424-ec2d7180-5647-11eb-94ee-a4a74582489c.png)
But as soon as I add a container at the end (before showing the window), the interface does not display correctly (you can see the unwanted padding at the bottom and right).
```nim
import nigui

app.init()
let window = newWindow()

let main_container = newLayoutContainer(Layout_Vertical)

let test_checkbox = newCheckbox(
    "Test checkbox")

let test_textarea = newTextArea()
test_textarea.editable = false

let progressBar = newProgressBar()

main_container.add(test_checkbox)
main_container.add(test_textarea)
main_container.add(progressBar)

window.add(main_container)
window.show()

app.run()
```
![Screenshot_1](https://user-images.githubusercontent.com/43933400/104551434-f18abc00-5647-11eb-9356-5a1388e77ae9.png)

Question: is this behavior normal? This does not show up on Linux, but does show up on all versions of Windows (7, 8, 10). Thanks