pdfview: evince integration before webview removal

Signed-off-by: Yannick GICQUEL <ygicquel@gmail.com>
main: replace WebKit preview by Evince

Signed-off-by: Yannick GICQUEL <ygicquel@gmail.com>
htmldoc: use prince pdf renderer if available

Signed-off-by: Yannick GICQUEL <ygicquel@gmail.com>