The abdication of Denmark's Queen Margrethe leaves Europe without a female monarch, but not for long -- a young generation of princesses born in the 21st century will be ascending to thrones across the continent in the coming years.Belgium's Princess Elisabeth, born in 2001; Catharina-Amalia of the Netherlands (2003); Ingrid Alexandra of Norway (2004); Leonor of Spain (2005); and Estelle of Sweden (2012) -- half of the hereditary monarchies on the Continent are likely to be led by a queen before