[UX] ReaderGesture: add back to previous location as an option (#4600)

See https://github.com/koreader/koreader/issues/4333#issuecomment-464361133

frontend/apps/reader/modules/readergesture.lua

@@ -66,6 +66,7 @@ function ReaderGesture:buildMenu(ges, default)
         {_("Previous page"), "page_jmp_back_1", not self.is_docless},
         {_("Forward 10 pages"), "page_jmp_fwd_10", not self.is_docless},
         {_("Next page"), "page_jmp_fwd_1", not self.is_docless},
+        {_("Back to previous location"), "previous_location", not self.is_docless},
         {_("Folder up"), "folder_up", self.is_docless},
         {_("Bookmarks"), "bookmarks", not self.is_docless},
         {_("Table of contents"), "toc", not self.is_docless},
@@ -201,6 +202,8 @@ function ReaderGesture:gestureAction(action)
         self:pageUpdate(-10)
     elseif action == "page_jmp_back_1" then
         self:pageUpdate(-1)
+    elseif action == "previous_location" then
+        self.ui:handleEvent(Event:new("GoBackLink"))
     elseif action == "folder_up" then
         self.ui.file_chooser:changeToPath(string.format("%s/..", self.ui.file_chooser.path))
     elseif action == "toggle_frontlight" then