Make cameras easier to find and harder to make
Add sanity-checked methods to item selections.

When fetching an item from the map (or from a vehicle) by using an index
into the list of items in that tile, we should verify the index is still
valid.

Prevents a couple of crashes relating to subscripting a non-existent
index past the end of a list.
Merge branch 'easier-cameras' of https://github.com/Coolthulhu/Cataclysm-DDA into merge2
Merge branch 'merge2'