Option to remember the `Separate` button state in the Curves view

Currently, the `Separate` button in the Curves view resets to OFF when Spine is restarted. Setting it to OFF by default is a safe measure for users who might forget to turn it off. However, if `Separate` is always reset to off, users who expect it to remain enabled may not notice that it has been disabled. They may then unintentionally modify curve handles in an unexpected way.

This is particularly easy to overlook when working exclusively in the Curves view without opening the Graph view, since the Curves view only shows the curve between the currently selected key and the next key. As a result, unintended changes to the shape of subsequent curves may go unnoticed until later.

Therefore, it would be a good idea to add an option to the Spine Settings that allows you to control this behavior. For example,

> Remember Separate state in Curves view

**Enabled**: Restore the last state of the `Separate` button Spine starts.
**Disabled** (default): Reset the `Separate` button to OFF on startup (current behavior).

This would preserve the current default while allowing users who rely on Separate to avoid enabling it every time they launch Spine.

**Related forum post:**
https://esotericsoftware.com/forum/d/30431-43-curves-feature-request

Start in the Curves view and Spine Settings, tracing where the Separate button is reset during startup. Done means a setting controls whether the last Separate state is restored, while the disabled default continues to reset it to OFF.

Written by the indexing model from the issue text.