Ambiguous tooltips on animation snapping toggles

### Tested versions

Present in v4.4.1.stable.nixpkgs [49a5bc7b6], but also in latest `master` (at the time of writing).

### System information

n/a

### Issue description

As I was trying to translate some Godot UI strings into Dutch, I realized that the snapping settings at the bottom of the Animation track editor have very ambiguous tooltips. The current tooltips are as follows:

1. Apply snapping to timeline cursor.
2. Apply snapping to selected key(s).
3. Apply snapping to the nearest integer FPS.

This doesn't make clear what is snapping to what, or how the toggles relate to each other; for example, does the first option snap something else *to* the timeline cursor? Or does it *make* the timeline cursor snap to something else?

After digging through the code for a bit, as far as I can tell, it should be something more like this:

1. Snap timeline cursor to nearest frame.
2. Snap new keys to nearest frame.
3. Round frame time to nearest integer FPS.

(The second one is changed a lot in particular; the current text speaks of "selected key(s)" but all code that actually evaluated this setting seemed to be about *creating* keys, whether from scratch or through duplication, rather than anything to do with selections.)

But I'm not 100% sure that this is a correct understanding of how these settings work, and so I decided to create an issue rather than a PR for now. Does this sound about right?

### Steps to reproduce

n/a

### Minimal reproduction project (MRP)

n/a

Start with the animation track editor's three snapping toggles and trace the behavior behind cursor movement, key creation or duplication, and FPS rounding. Compare those behaviors with the current and proposed tooltip wording; done when each tooltip clearly identifies what is snapped and the text matches the implemented setting.

Written by the indexing model from the issue text.