flatpak use of January 1st 1970 as date creates problems for KDE software

[Albert Astals Cid]

flatpak mounts all files as if created on January 1st 1970.

This has the unfortunate effect that when we add new plugins to a flatpak (i.e. we just added markdown preview support in kate), they are not seen because ksycoca (kde plugins cache) doesn't feel the need to regenerate itself (the sycoca file for flatpak is written "outside" flatpak container and thus has a newer date).

I remember talking about this probably a year ago at least with Aleix, but i guess we did not come up with any solution since it's still broken :D

I don't know much about flatpak, but can we make it so the date is not always January 1st 1970 but some of these?
 * the date the flatpak was built
 * the date the flatpak was installed

I've seen patches flying around for other stuff (VLC at least i think) so this hardcoded date issue seems to be hiting more people than just KDE.

Cheers,
  Albert