Avoids concurrent access to _addons and _displayedAddons.

- remove the + and - buttons, and add a single install button
  instead. Current implementation of uninstall / install has a
  toggle behaviour, so having two buttons was misleading.
- add a fetch button, to load the online repository on request only,
  as done in the qt interface.

So, create a method using QStringList and not input_item.
This should be cleaner in the calling sites, and avoid
calling input_item_new from Qt code

And there was no need to use the Item version, and the MRL was bogusly
added to RecentsMRL