Readd closebutton in files_pdfviewer

Also used proper appending instead of poor-man's appending
(`$("#content").html(oldcontent+'<iframe style="width:100%;height:100%;display:block;" src="'+viewer+'" />');`)
so `$(document).ready()` is still in memory after we loaded a pdf, to make it possible to load another pdf.

#915 

cc @jancborchardt @tanghus @DeepDiver1975