Fix insertion position for new controls when append or prepend options are enabled

[lucasnetau]

This PR fixes the following issues:

• Dragging a control onto the stage before a prepended field or after a appended field would show the drop placeholder and then silently cancel the drop on cursor release. We can fix this by relocating the placeholder with the jquery-ui.sortable change event
• Adding a field by clicking on the control would insert the new control after a prepended field. Correctly set h.stopIndex to ensure controls are added before the prepended field
• When sorting stage controls using the mobile sort buttons the sort handler did not take into account appended/prepended fields

Partial fix for #665

[kevinchappell]

much needed polish, good job

[github-actions]

🎉 This PR is included in version 3.10.3 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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