Rename unselect to deselect

[madmiraal]

As originally identified in #28558, "unselect" should be "deselect".

This PR:

• Renames ItemList's methods unselect() and unselect_all to deselect() and deselect_all()
• Renames GraphEdit's signal node_unselected() to node_deselected()

and for consistency and to make it clear that it deselects all items:

• Renames FileDialog's deselect_items() to deselect_all()

Part of #16863

[akien-mga]

Thanks!