Navigate and edit caption parts with shortcuts

[KennethWussmann]

There should be keys to navigate the added caption parts.
There is already Arrow Up which will edit the last added part, similar to Slack or Discord.
This solution would replace it with one that offers more flexibility.

Instead of immediately editing the last part, Arrow Up and Arrow Down are basically a cursor, focusing on one of the parts, like when using Tab Navigation.
When then pressing enter on a focused item it should start editing this.

That effectively means that editing the last part got a little slower with two key presses instead of one, but fair trade for better flexibility.

When a part is added:

• The cursor that highlights the selected part hides. When pressing Arrow Up now, you'd highlight the last part again. So the cursor position reset.

When the part you are editing is deleted:

• Same behaviour like addition

When editing is aborted (ESC):

• Edit mode is exited
• Cursor stays visible on same part

When editing is saved (Enter):

• Edit mode is exited
• Part is updated
• Cursor stays visible on the same part