Change flipping to use the joint bounding box of the selection

[henryksloan]

Closes #316

• Replaced the FlipLayer message with a new FlipSelectedLayers message that takes a FlipAxis enum
• Changed the Select ToolMessages to use this new message
• Implemented the FlipSelectedLayers message
  • Collects the max and min X or Y coordinates of each layer's bounding box
  • Aggregates the max's and min's to get the extrema of the bounding box in the given axis
  • Takes the average of these extrema as the flipping axis
  • Scales each layer by -1. along the axis of flipping, and translates it to the same distance on the other side of the axis

Caveats

• The flipping is still in canvas space, similarly to alignment. This is because the current bounding box implementation (I believe) exclusively works in canvas space.
• The code related to joint bounding boxes is quite repetitive and verbose between the flipping and alignment procedures
  • Add a bounding box around selected layers in the Select tool #282 is supposed to add a merging function between two bounding boxes, but it is not yet ready to review
  • I could add the function for use in these methods, and it could be reused when work on that PR continues
  • Either way, that should probably be saved for another PR

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Bug

[Keavon]

Before:

After horizontal flip:

Flipping it back restores it to the original design.

After vertical flip:

[Keavon]

The above bugs arise from their rotated coordinate system when they are created. We'll have to fix that later.