Throwing bugfixes

Removed an unused client verb, changed a usr to src where they're the same mob, and changed a type == check to istype().

Allow turning on throw mode when clicking the UI button (hotkeys already allow you to do this).
Disallow throwing when not on a turf.
Fixes #4823

Make storage items close out the onscreen stuff when they get thrown.
Fixes #5296
Merge pull request #5776 from MrPerson/throwing_changes

Throwing bugfixes