Show vertical marker at original position when back from link (#3669)

ReaderLink: make all links be a table (they were a table for PDF,
but a string for CRE) for clearer code. Also have location_stack
store them as tables, with additional properties.
Get original position of link source (and verify it is valid)
so we can show a marker there.

Also:
Hold on "Go back to previous location" to clear location stack.
Resists "Swipe to go back" when previous locations stack has just
become empty, and show a notification.
Fix wrong links with Swipe to follow nearest link on PDF documents.

frontend/apps/reader/modules/readersearch.lua

@@ -103,7 +103,7 @@ function ReaderSearch:onShowSearchDialog(text)
                         end
                     end
                     if valid_link then
-                        self.ui.link:onGotoLink(valid_link, neglect_current_location)
+                        self.ui.link:onGotoLink({xpointer=valid_link}, neglect_current_location)
                     end
                 end
                 -- Don't add result pages to location ("Go back") stack