(singleton) Avoid warnings at compile with gcc/clang

Also update CuteLogger to remove compile warning
(logger) Do not translate log messages anymore
(search) Emit signals with status messages and icons
(taskmonitor) Display results entries only at first notif
(searchprovider) Refactoring on methods name
(searchprovider) More refactoring
New version number for next release: 0.3
(cmake) Display a warning if OAuth key have not been set
(main) Delete all singleton instances in the same place
(main) Delete useless data from user settings
(settings) Remove useless entry about API url
(oauth) Improve events monitoring

Remove useless multiple page content, keep only the webBrowser
Override link to prevent errors when keys have not been set
(oauth) Ad an action to log in / log out from/to Beatport API
(oauth) Improve messages displayed in status bar
Add documentation