Implement Cut Selected Keys in AnimationPlayer

[CookieBadger]

Following up on #87250, I added an additional option to "cut" the selected keys, i.e. both copy and remove them at once. This follows the way that cutting is implemented in the SceneTree.
Same as copy and paste, the option is accessible both through the right-click menu and the Edit menu, as well as with the usual shortcut of CTRL+X.
While not necessarily a complex feature, the lack thereof would definitely confound some users, especially since pressing the familiar CTRL+X in hopes of cutting the selected keys (since CTRL+C, CTRL+V work as well) would remove the currently selected node in the scene if this shortcut were not implemented in the AnimationPlayer.
The implementation works both in the default and the Bezier editor.

[CookieBadger]

This should also complete #3419.

[Calinou]

Tested locally, it works as expected. Thanks a lot for improving animation editor usability 🙂

Code looks good to me at a glance, but @godotengine/animation should take a second look just in case.

[akien-mga]

Thanks!

[Kartopod]

Oh my god i love you

[holzmb]

@CookieBadger what a legend.
one question, do you know if it would be possible to implement selection of all keyframes at certain time?
for example you want to select all keyframes across multiple tracks at exactly(or around) 1 second mark.
i think it's called master keyframe in every animation tool.
i'd literally pay you.