Make resolution selector in options into a dropdown menu

[cooljeanius]

SuperTux version: 0.6.3
System information: macOS Big Sur

Expected behavior

Clicking on the "Window Resolution" item in the "Options" menu displays a dropdown menu from which I can select my display's resolution (3072x1920)

Actual behavior

Clicking on the "Window Resolution" item in the "Options" menu simply switches to the next resolution available, making it hard to figure out how many more clicks it'll take to get to my desired resolution

Steps to reproduce actual behavior

1. Open SuperTux
2. Open the "Options" menu
3. Click the "Window Resolution" item

[Semphriss]

I support, there is no need to scroll through all possible resolutions.

I recommend using the recently added string selection menu.