If we detect a problem that requires intervention during loading,
e.g. we find unknown elements because we're loading a file created
by a newer version, default to read-only mode.
Read only mode (1) refuses to apply changes, (2) refuses to add
changes to the undo/redo list, and (3) disables Save/SaveAs. The
mode is indicated in the title bar.
Also, flipped the order of items in the title bar so that "6502bench
SourceGen" comes last. This allows you to read the project name in
short window title snippets. (Visual Studio, Notepad, and others
do it this way as well.)
