[orx-gui] add windowResizable argument to WindowedGUI

[hamoid]

This works well for vertical resizes of WindowedGUI, but less ideal when making the window wider, because the width of the GUI is only set when the window is created, not when the window is resized.

Simple solution

In Gui.kt set

this.width = 100.percent // instead of appearance.barWidth.px

for container and sidebar ONLY WHEN using WindowedGui.

Potentially better

Stretch all inputs to match the window width (with some right margin). That requires more effort.