Doomsday Engine

Code style change: use of underscore in private members.

Rationale: Previously the underscore was suffixed to private member names.
This results in ugly code such as:

member_->func();

Now all private members use the underscore as a prefix, which corrects the
issue:

_member->func();

Also, now the uglier case occurs when private members are accessed through
pointers/references (which should occur rarely):

somePtr->_member;
  someRef._member;