ListTile's onTap to select

[MrLightful]

Blocked by #21 -- review & merge it first

Refactors item builder, so to use ListTile's onTap callback to select items. This improves User Experience, as the checkbox by itself is very small and proved to be sometimes hard to trigger by click. So instead, now, the user can click anywhere in the tile to trigger selection.

https://github.com/Mindinventory/drop_down_list/assets/14923964/6520c522-fa25-4104-9026-ecafa7d03efd