[BezierEditor] Fix fadeout issue

Steps to reproduce the issue, introduced by #7235:
1. Start an Inline Timing Function Editor
2. Make the function invalid, like `cubic-bezier(1.000001, 0, 0, 0)`
3. Make it valid again
4. Make some other valid changes (like deleting the additional zeros) while the fadeout is still running

Result: The hint is immediately hidden
Expected: The fadeout should continue

This PR makes the code a bit cleaner (using `editor.hint` object for everything that has to do with the hint) and uses `.hide()`, suggested by @zaggino.