A Bucket will no longer jump to be centred around the cursor/finger when
dragging.
oldTouchPos is global to the class so...
New 'Value' class.
Use Value in the Bucket class.
Use the Value throughout the code.
Code cleanup.
Fix for the case where a bucket may stick to the line incorrectly.

For example,

1. Place 1 and 2.
2. Move 2 to the left of 1
3. Place 3 between 1 and 2

Final order: 2 3 1