Pocketbook: File handling OS integration (#6617)

* Show opened file in task manager
* Remember file to open on power-off
* Perform file associations

frontend/apps/reader/readerui.lua

@@ -583,6 +583,7 @@ function ReaderUI:doShowReader(file, provider)
         local _, filename = util.splitFilePathName(file)
         Screen:setWindowTitle(filename)
     end
+    Device:notifyBookState(title, document)
 
     UIManager:show(reader)
     _running_instance = reader
@@ -692,6 +693,7 @@ end
 
 function ReaderUI:onClose(full_refresh)
     logger.dbg("closing reader")
+    Device:notifyBookState(nil, nil)
     if full_refresh == nil then
         full_refresh = true
     end