[Users] Some Message Part 'Open With' Dialogs Missing 'Remember This' Option

[Dustin Miller]

3.17.4 on Lubuntu 18.04

The "Open With" dialog windows for some message part types are missing
a "Remember this" box, which means you can't use these windows for
making a permanent change in the default program that is used for
opening that part type.

For instance, the message part type 'message/rfc822' does have this
box, but the types 'text/plain' and 'text/html' do not. This is
consistent whether using the keybinding, R-click context menu, or the
main menu.

I am not sure how many different message part types there are or how
many different times/places the "Open With" windows associated to these
are included in the code, but I think it would likely be a helpful
addition if every instance of these would include a "Remember this" box
in order to change the default.

Shall I file a bug for this? If so, for the 'Importance' category,
should I go with 'Minor' or 'Enhancement'?

Thanks,
Dustin