macosx: lock access to addon_entry_t
macosx: addons manager: fix crash when window is repeatedly opened
macosx: addons manager: restructure UI and fix some problems

- 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.
macosx: addons manager: process the callback on the main thread

Avoids concurrent access to _addons and _displayedAddons.
macosx: addons manager: add l10n for download button
lua: meta: add descriptor/scope
finder/fetcher: always use scripts doing local access
rename art_finder as meta_fetcher
deprecate art-album variable
update meta request API to allow overriding
Qt: update art requests to use updated API
lua: include cleanup
Qt: cosmetics on RecentsMRL and Open classes
Qt: save recents on quit()