Try to load textures from main data directory if they aren't in 'textures' dir.

This fixes the issue reported by Manfred. This might not be the best option available,
but we don't want to break anything for people that are happy with textures in
main data directory.

CCMAIL: manfred.kitzbichler@gmail.com
CCMAIL: kstars-devel@kde.org
CCBUG: 297210