Lyrics: crash when copy+undo+paste

[matevz]

Steps to reproduce:

1. add some notes and some lyrics
2. copy some syllables
3. undo
4. paste syllables => CRASH

It smells as if the document which the clipboard is relying on was deleted before pasting is made. Doing copy&undo&paste on notes in staff doesn't crash though 🤔

Suggested solution:

• when copy is triggered, CLONE the document and store it in memory along with the references of elements to be pasted
• this makes copy&pasting oblivious to future document changes
• but probably requires that each clone() also returns a map of QMap<void*,void*> old -> new so that you can reference the elements to be pasted somehow. This feature would also be useful in the UI when calling undo/redo so it can remember the selection.