Split up html5sortable.ts into smaller modules

[lukasoppermann]

It would be good to split the html5sortable.ts file into smaller modules to make it easier to test them and understand the code.

Important would be:

• try to avoid using globals as much as possible (always inject arguments in functions rather tan relying on options)
• try to include typescript definitions for the functions
• try to include tests for the new parts
• try to see if we can generalize a function so that it can be reused for other parts
• include comments and documentations in the module files
• try to make the function naming convention more straight forward, logical and homogeneous

Good candidates are:

• findSortable
• findDragElement
• _getDragging & rename to make clear what the difference between findDragElement and getDragging is

[lukasoppermann]

Hey @kaffarell if you have time to ship in here that would be awesome.