Unified API to make UI components draggable

[Nibba2018]

Many UI components such as Panel2D, ComboBox2D, TabUI etc implement boiler plate code to make themselves draggable. It would be helpful and cleaner to have a separate common method, preferably part of the base UI class to replace the said boiler plate code.

Describe the solution you'd like
A common UI method that helps replace the boiler plate code for making UI components draggable.

Additional context
Here's one such implementation for ComboBox2D. A similar implementation is done for TabUI as well.

[skoudoro]

I agree, I think this is part of creating a layout system.