Store BulletShape source file name and content hash

Alternative to !1374 (closed).

This is a second step towards platform independent (portable) navmesh disk cache and a first step to reduce its size. Here we prepare a file content hash for each bullet shape. In combination with file name this will later be used as a part of navmesh disk cache key. To be more precise this data will be stored into a separate table in the navmesh.db and the navmesh tile key will contain only a sequence of identifiers (int64 numbers) to these shapes.

To get file has we read the full file first and then reset position back where it's started. I expect that it's ok to read all mesh files until the end. For nif hash is stored as a separate field. For other formats to avoid changing return type I store it into the osg::Node via setUserValue. Unfortunatelly osg doesn't support std::uint64_t type there so I had to store it as a string.