Allow sloppy dropping of items on to Inventory/Stash/Vendor

Fixes #1060

Other fixes:
- Cached drag icons now have abbreviated quantities like slots do
- Equipment slots are now highlighted regardless of drag source
Merge pull request #1065 from dorkster/fixes

Allow sloppy dropping of items on to Inventory/Stash/Vendor
Merge remote-tracking branch 'upstream/master'