Don't use gnome-desktop thumbnail preview creator - closes #85

[fossfreedom]

Description

Workaround the deemed insecure usage of gnome-desktop thumbnail preview generator.

By the time this part of the code is reached, all the file & mime-type validations have passed. So just create a image preview using the standard pixbuf routine.

Note GCC uses the GTK4 version of gnome-bg thumbnail generator that does not use the deemed insecure bwrap routine hence why it works just fine.

Submitter Checklist

• [ ] Squashed commits with git rebase -i (if needed)
• [ ] Built budgie-control-center and verified that the patch worked (if needed)

[fossfreedom]

Debian/Ubuntu have decided to handle this via apport changes to bwrap. Suggest other distros using apport similarly need to-do the same. As such this PR isn't needed.