Inspector pivot coordinates and mobile-friendly skew

[StickmanRed]

Description

This PR adds:

• "Pivot X" and "Pivot Y" values to the inspector, both desktop and mobile. These are expressed in relative coordinates to the selection or single clip (before translation, scale, shear, and rotation are applied).

• Pivot snapping to the selection's corners, edge midpoints, and center. It can be disabled by holding Alt while dragging the pivot. The pivot snap distance is currently set to the pivot handle's radius.

• The ability to skew by dragging horizontally. This gets overridden when any modifier keys (Ctrl, Shift, Alt) are pressed. Currently set to skew within a vertical distance of 10

z_0001.mp4

Todo

• Add icons to mobile inspector pivot X/Y inputs

Testing

Try entering different values for selection pivot X/Y in the inspector. Then move, scale, shear, and rotate the selection using the canvas tool: the values for pivot X/Y should not change.
Drag the pivot point close to the center, one of the corners, or one of the midpoints: at a close enough distance, the pivot should snap to these corners. This can be verified by viewing the pivot's exact coordinates in the inspector.

Checklist

• This PR does not make sense to split up into smaller PRs. (I probably should've moved the skewing behavior to a separate PR but oh well)