Additional commands added to the timeline need to go below the last selected node, not the bottom of the timeline

### OS

_No response_

### Godot Version

4.X

### Plugin Version

Lastest commit from the repository

### Issue description

Whenever a new node is added to the timeline, it needs to go below whatever was last selected, not at the bottom of the entire timeline. If I have a set of commands 1, 2, 3, 4 and I want to insert command 5 between 2 and 3, I would have to insert command 5, then manually drag it up to where it's between 2 and 3.

This might not sound bad, but let's say I have command 200 and I want it between commands 102 and 103. Now I have to move upwards of 100 commands just to manually drag it to where I want it to be. Simply letting me just click command 102, inserting a new command, and having it automatically be the new 103 would be way better.

### Steps to reproduce

Native to the engine, no steps needed.

### Workaround

_No response_