Fix two related bugs with svg support

1. Do not treat .svgz as archives
2. Fix directory not loaded when started with a .svg as argument

Posted for review on https://git.reviewboard.kde.org/r/111362/ but the changes
did not get reviewed. I am committing them nevertheless because I want the bug
to be fixed in rc2 and my personal testing did not spot any issue.

FIXED-IN: 4.11.0
BUG: 207115
BUG: 321825