[zeditor] Map Copypasta Memory Leak

Steps to recreate:

1. File->New
2. Press Ctrl+Shift+ESC to bring up Task Manager
3. Highlight zeditor.exe and view memory usage
4. Select combo #12
5. Ctrl+Click the screen; Observe entire screen turns from captain blue to tan
6. Edit->Copy
7. Do the following many times:
   
   • Edit->Adv. Paste->Paste To All
   • Click 'Yes' to paste to all screens on the current map
   • Edit->Maps->Delete Map
   • Click 'Yes' to clear this entire map
8. Observe memory usage of zeditor.exe has trended upwards

onPasteToAll() is in zquest.cpp

File->New appears to clear the undo/redo stack.