Recalculate "nextLineID" when setting nextLine to mainLine with ctrl+space hotkey

[Bhajneet S.K.]

Describe the bug
Recalculate "nextLineID" when setting nextLine to mainLine with ctrl+space hotkey

To Reproduce
Steps to reproduce the behavior:

1. Open shabad
2. Jump to next line
3. Use ctrl+space hotkey to set it as main line
4. Selected line is now the main line and next line

Expected behavior
Next line would be mainline+1 if exists, else delete next line

Screenshots