Added the recovery file check when a project is opened, and the GUI
elements for choosing which file to use.
If a recovery file exists but can't be opened, presumably because it's
open by another process, offer to open the project read-only. (This
is a generally good idea, but we don't hold the project file open
in normal usage, so it only works when auto-save is enabled.)
After making a choice, auto-save is disabled until the first manual
save.
One thing we don't do: if we find a recovery file, but auto-save is
disabled, the recovery file won't be deleted after the user makes a
choice. This might be a feature.
Updated documentation.
(issue #161)
fca742e890