Slider: move by at least the next increment when using fixed_decimals

[0x53A]

fixes #7065

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7066-fix-slider-keyboard-move-max-decimals
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[emilk]

Makes sense! Can you please add a test for this though (using egui_kittest)? I don't think you need a screenshot test, but it would be nice with a test that sets up a value, then injects a arrow key, and then reads back the value to see that it changed by the expected amount.

[0x53A]

i've manually checked that the test fails on main and succeeds on the branch

[emilk]

Thanks for adding that test!