The new display name code handles _dark and _svg like an property
or extension. It strips both from the back of the base filename
and adds them in brackets to the display name as (SVG), (dark)
and (SVG + dark).

This way we can drop the special handling for breeze_dark and
sifr_dark display names.

Change-Id: I0ebc8392582d7240b51528078de9d81b36e5a9e9
Reviewed-on: https://gerrit.libreoffice.org/62788
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>