file-undo-manager: rename undo_redo_flag

Nautilus undo manager previously relied on a stack-like implementation of its
operating state. The stack operations (pushing and popping) were removed in a
previous commit due to possible unexpected behavior and errors. The flag alone
now has an inadequate name for what it represents.

Rename the flag to "is_operating", since it is set when the manager is
performing an undo / redo operation and unset otherwise. Rename the associated
getter function accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=759850