support goto relative page number

We can input relative page number now in the reader goto dialog.
Goto "+4" will page forward 4 pages and goto "-4" will page
backward 4 pages.

This implements #1437.

frontend/apps/reader/modules/readerrolling.lua

@@ -331,6 +331,14 @@ function ReaderRolling:onGotoPage(number)
     return true
 end
 
+function ReaderRolling:onGotoRelativePage(number)
+    if number then
+        self:gotoPage(self.current_page + number)
+    end
+    self.xpointer = self.ui.document:getXPointer()
+    return true
+end
+
 function ReaderRolling:onGotoXPointer(xp)
     self:gotoXPointer(xp)
     self.xpointer = xp