5098: refactor old code to use User.progress

Reapply from trunk.
    Now that #7061 is done, all the old code should no longer use
    User.begin_.../step_.../end_progress directly any more, preferring
    the context manager instead.
The only reject is on gramps/gen/merge/diff.py that doesn't have
any progress callbacks on gramps40. I'm leaving it untouched.

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/maintenance/gramps40@23097 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
Revert "5098: refactor to use User.progress"

OOPS. This was tested only with "runtest.sh", but turns out
things are broken with GUI :-) Reverting for now @gramps40,
will investigate on trunk!

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/maintenance/gramps40@23098 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
7061: rename gui.user.User.progress to ._progress

it shadowed the gen.user.User.progress (the context manager),
breaking the ``with user.progress() ... as step:'' idiom!

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/maintenance/gramps40@23103 4ae1f11a-8b86-4847-b8af-ab372f36d1fd