libdeng2|Record: Added an explicit setter for char const*

Evidently some compilers (MSVC at least) interpret a zero-length,
c-style string as bool, consequently resulting in a NumberValue
rather than a TextValue being written to the Record.
Windows|Debug|libgui: Don't assert GL operation success

At shutdown, after restoring the original display mode, there was an
OpenGL error occurring that caused a crash (assert dialog => recursive
repaint?).
Fixed|libdeng2: Copying a Record

When making a copy of a record (or assigning one), subrecord ownership
was not replicated correctly in the resulting instance.

Added a test case in test_record to verify this behavior.
Tutorial|Client: Final tutorial step only needs the "Done" button
Merge branch 'master' into savegame-refactor
libdeng2|SavedSession: Ensure contents are up-to-date when recognizing .save files

As of this commit, .save files are recognized by the repository,
the session metadata is parsed from Info, SavedSession is updated
accordingly, game menu is updated by proxy via SaveSlots, sessions
are determined as loadable by the game and file duplication works
as expected by Hexen.

Loading of the saved session file currently fails with a caught
Block::OutOfRange error, presumably when attempting to deserialize
the ACScript data...