Fix editor spin slider remaining editable if set read_only during and edit and fix related animation player crash

[spanzeri]

This fixes a crash in the animation player.
The easiest way to reproduce it is to open 2 scenes, one with an animation player and any other one.
First select and focus the line edit for the snap step, then switch to another scene.
Two issues are noticeable:

• The editor spin slider remains editable, even though the animation is no longer selected and the spin slider has been set read only;
• The animation editor crashes trying to update the value of the snap on an animation that is no longer valid;

The changes on editor_spin_slider makes it so the line edit is correctly unfocused and un-selected if it becomes read only mid-edit.
The animation_track_editor check is just a sanity check and is not strictly necessary with the editor spin slider changes

[spanzeri]

Thank you for the suggestions!

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.0.3.