Visualize spawn interval in editor dialog

[SimonN]

Lix 0.10.0

In the editor's Constants dialog you can select the spawn interval for the level. There is a number-picking widget to pick an integer between 1 and 96 inclusive.

Idea: Next that widget, visualize the chosen spawn interval. Draw a still image, or perhaps even an animation, of some lix falling from a single hatch and walking over flat ground.