[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 move to
QFileSystemModel, clean up file filtering to be compatible with Qt5 and
QFileSystemModel.