[RFA 03/22] Use scoped_restore for ui_file

[Tom Tromey]

>>>>> "Simon" == Simon Marchi <simon.marchi@polymtl.ca> writes:

>> FWIW Mozilla uses an Option class for this kind of thing.
>> It works like:

If you happened to search Firefox looking for this, it's called Maybe,
not Option -- Option is the Rust name for this.

Also I thought it was worth pointing out that this has made it into
C++17 under the name std::optional:

http://en.cppreference.com/w/cpp/utility/optional

Tom