[Pre-Port] Change toAscii > toLatin1

Forward compatible changes.
[Pre-Port] QPixmap for QIcon constructor

Forward compatible changes.
[Pre-Port] SetShown > SetVisible

Forward compatible changes
[Pre-port] Replace Qt allocation with stdlib

Forward compatible changes.
[Pre-port] Replace QAbstractItemModel::reset()

Forward compatible changes.

reset() was deprecated in lieu of beginResetModel() / endResetModel()
[Pre-Port] Message handling changes for Qt5

Forward compatible changes.  Uses compile time branching to target Qt5
only.
[Pre-Port] Changes to file opening

Forward compatible changes.  First, the dragging or double clicking of a
file gets broken when porting to Qt5 because of changes to QUrl.
Second, there is code for handling 8.3 filenames, but as far as I can
tell it's unnecessary.
[Pre-Port] QDirModel > QFileSystemModel, filters

Forward compatible changes.  QDirModel is deprecated, so replace it.
Change filters so that they will work when moving to Qt5.
[Pre-Port] Hide unused NifSystemLocale

Forward compatible changes.  NifSystemLocale subclasses QSystemLocale
which became private API in Qt 5.   It was not used anyway, so it's been
commented out.