"Are you sure" dialog for deleting entities should accept a return keypress to mean "yes"

[vts]

The "are you sure?" dialog for deleting entities should accept a return keypress to mean "yes". It's annoying to have to reach for your mouse and point and click "Yes" when you want to quickly delete an entity. Alternatively (or additionally), as proposed by Mythos_Ruler, we could also have another press of the Delete key to accept the dialog.

[Deiz]

In 12503:

Handle exit and return hotkeys in message boxes. Fixes #1319.