Blender Git Loki

File/Asset Browser: Don't deselect other items when dragging

Basically this enables the select-tweaking behavior as per the
guidelines:
https://wiki.blender.org/wiki/Human_Interface_Guidelines/Selection#Select-tweaking.

We use this in most other other editors that allow selecting and
dragging multiple items. But besides the consistency improvement, this
is important if we want to support dragging multiple assets (or files)
in future. We want to support this at least for dragging multiple assets
into an asset catalog for the upcoming asset catalog UI.