Fix SVG icons for import/export preferences #2956

The SVGs for opening the import/export preferences wizard inside the
preferences dialog have erroneously been taken from wizard banner icons,
which have more details and are larger than the actual icons to be used.
This leads to either blurry icons or icons that are too large within the
preferences dialog.

This change replaces the SVGs with ones that are resized (from 16x16 to
24x24) from the existing import/export wizard tool icons to conform with
the existing PNG icons.

Fixes #2956

Author: HeikoKlare