Reader menu: adds "Open previous document" (#4056)

* Reader menu: adds "Open previous document"

Allows for quick switching between 2 documents, and keep
symmetry with FileManager menu "Open last document".

* Use "Last:" and "Previous:" when Show filename

frontend/apps/reader/readerui.lua

@@ -663,4 +663,11 @@ function ReaderUI:reloadDocument(after_close_callback)
     self:showReader(file, provider)
 end
 
+function ReaderUI:switchDocument(new_file)
+    self:handleEvent(Event:new("CloseReaderMenu"))
+    self:handleEvent(Event:new("CloseConfigMenu"))
+    self:onClose()
+    self:showReader(new_file)
+end
+
 return ReaderUI