Function getVisualEdges

Returns the "visual" edges that can be dragged.

Visual edges are a combination of all edges shared by frames that must be moved together.

For example, if we have two frames A and B, they share the center edge, and the function would return this single edge (unless includeWindowEdges is true).

┌──┰──┐
│A ┃B │ returns   ┃
└──┸──┘
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Or in this example, we would get two edges:

• The one shared by A, B, and C, because no frame edge sharing this edge can without also moving the other frames touching the edge.
• The one shared by A and B.

┌──┰──┐
│A ┃C │              ┃
┝━━┫  │ returns   ━━ ┃
│B ┃  │              ┃
└──┸──┘
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And here, we get four edges, because all four can be moved on their own (moving the edge would only affect the two frames sharing that edge).

┌──┰──┐
│A ┃C │             ┃
┝━━╋━━┥ returns   ━━ ━━
│B ┃D │             ┃
└──┸──┘
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