Replace arrow images with icon themes carets in the Solr Search facets

There are 4 images in https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-core/xwiki-platform-search/xwiki-platform-search-solr/xwiki-platform-search-solr-ui/src/main/resources/Main/SolrSearch.xml#L47 that are used as background in arrows.png.
One advantage of having the arrows as font, their colors can be easily changed to match the used color theme.