Drop target support

This module adds drag-and-drop file support to Odoo's backend interface. When a user drags a file over a form view, a visual overlay appears showing where the file can be dropped.

• In form views, users can drop any type of file onto the form area. The file is uploaded and stored as an attachment linked to the record currently open.
• If the record has not been saved yet, the overlay warns the user to save the record before dropping a file.
• After a successful drop, the form reloads so the new attachment appears in the chatter.

The module works by extending the standard form controller with a reusable drop-target behaviour. It listens for drag and drop events on the page body, shows an overlay only when the pointer is over the form, and hides it when the pointer leaves the window.

Technically, the uploaded file is created as an ir.attachment record with the file name, binary data, and a link to the current model and record. The module does not add any menus, database models, or server-side business rules of its own; it is a base layer meant to be reused by other modules that want to handle dropped files in different ways.