Transform cage skew sliding triangle gizmos

[Keavon]

As of #2251, you can currently skew by holding Ctrl when dragging an edge of the transform cage. I'd like to add these triangles that appear when hovering over an edge's resize click target. Only the edge for which you're hovering should show the pair of triangles. Triangles are the same dimensions as the squares (7 x 7 px), but with a vertex at 7 / 2 = 3.5 instead of the square's corners. They are located 4 px away from the square. After adding this, we can remove the Ctrl shortcut which isn't needed anymore since this is easier and more discoverable. However, when dragging a triangle, Ctrl can be assigned to allow the dragging of the entire edge instead of just sliding it. Meaning it's allowed to follow the cursor along both the local X and local Y axis, not just sliding along the local X or Y axis (whichever edge is being dragged). If Ctrl is released while dragging, it should return to its previous sliding-only position, as if the user had never held Ctrl but merely moved the cursor around, only sliding it along the local axis. The hints, during the skew sliding, should indicate that Ctrl unlocks the slide.

[mTvare6]

Will be fixed by #2300