Configure Thunderbird to use the KDE file picker on Plasma

[TraceyC77]

By default, Firefox and Thunderbird use the Gnome file picker. On Plasma, this looks out of place. It also sometimes means that bugs with the Gnome file picker get reported to kde.
(I ran into that particular bug myself just now with Thunderbird)

The KDE packaging recommendations say this for Firefox:

Set widget.use-xdg-desktop-portal.file-picker=1 in about:config to make Firefox always use XDG portals for file pickers, which will make it use KDE's file picker instead of the GTK one.

I would request we do this for Firefox and Thunderbird, when installed on Plasma

[davidjharder]

It should be set for Firefox
https://github.com/getsolus/packages/blame/5868403fa923b5c57ae427907f87ef7204b2610d/packages/f/firefox/files/prefs.js#L49

But we are missing a similar line for Thunderbird

[TraceyC77]

On both my laptops, Firefox had been using the GTK dialog. I'll re-test it with a 4.7 ISO