Undo functionality

[griffi-gh]

Experimental undo functionality, needs testing

closes #139

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TODO (in rough order of importance):

• Reset undo stack when loading another level
• In-memory compression (large levels can create ~10mb of data per undo step without it)
  • Asynchronous compression using a background worker thread
  • Incremental snapshots (since changes are minimal, find a way to compress just the deltas)
• Fix icon visibility
  • Only show while in sandbox mode
  • Gray out if nothing to undo
• Add checkpoints on events:
  • Object created
  • Connect/disconnect
    • "Connect all" shortcut (Shift+E)
    • "Connect" shortcut (the F key)
    • Connect (only using the ui button)
    • Inter-layer connect (only using the ui buttons)
    • Disconnect
      • (ui button)
      • (Disconnect shortcut?)
  • Object moved
  • Object props changes?
  • Pasted partial
  • Deleted object
  • (Lock/unlock object?)
  • Change layer
• Redo (keep data when undoing for restore)
• Keyboard shortcut (ctrl+z)
• Better undo icon (current one doesn't fit in)
• Performance, check and try to improve it if needed (possible stutters on checkpoints!)
  • Currently takes up to ~50ms to save a snapshot of a decently sized level on my PC

Etc:

• Should level properties be undone/preserved as well?
  • (currently they are undone as well as object changes, since im saving the entire level data to ram)