Don't let back arrow be re-colored to monochrome

The back button uses a custom content item with the icon being provided
by Kirigami.Icon. Unfortunately, Kirigami.Icon currently suffers from an
issue that causes colored icons that end with "-symbolic" to be forced
to monochrome. See

There is an open merge request to fix that
(frameworks/kirigami!511), but
the fix is in Frameworks, which means it will only get to people who use
rolling release distros or whose packagers backport the fix.

This commit targeted at the Plasma/5.24 branch only is a local
workaround intended to alleviate the situation for LTS distro users who
are less likely to get the frameworks fix, once it's merged. The master
branch doesn't need the workaround since we can hope  that the
Frameworks fix will be merged before Plasma 5.25.

CCBUG: 451538
2 jobs for Plasma/5.24 in 1 minute and 35 seconds (queued for 47 seconds)