that to try to find them if we can't find them by absolute path.
This is intended to make everything still work if you copy an
entire directory tree of files that import each other.
Also add a mechanism to not paint the scene if we're not sure it's
consistent; otherwise got some crashes on startup. And disable both
text and graphic window when displaying a modal dialog, wasn't
doing that always.
[git-p4: depot-paths = "//depot/solvespace/": change = 1808]
Jonathan Westhues