Triangular mode

Add an alternative triangular mode for creation of hexagonal and triangular patterns.

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Current progress:

• [x] Snapping
• [x] Grid
  • Could be better though, seems to be missing one column on the right.
• [ ] Rulers during movement
  • (should have three axis at 60 degrees to each other)
• [ ] Arrow-key navigation
  • (can't be done perfectly since only 4 keys, but a best approximate can be done)
• [ ] Switching between square and triangular mode
  • [ ] UI toggle (where best to put?)
  • [ ] Keybind toggle (what key combination?)

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Are there any other aspects I might have missed that would need to be adapted for a triangular mode?