UI: Add box select to preview

This change adds the ability to box select by clicking and dragging from
an empty part of the preview.

Shift + Drag add any items in the box to the selection. Alt + Drag will
remove items in the box from the selection. Ctrl + Drag inverts the
selected state for items in the box.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f