Move multi-select actions out of context menu

[Maxim Stykow]

The actions to remove or confirm multiple attributions in attributions view are currently hidden inside context menus. In order to make these features more transparent, add them as icons above the attribution list similar to how an email client works, for example.

Also add an option to select/de-select all attributions.

Finally, convert the filter select to a menu in order to not need more vertical space for the new actions, filters, sortings etc. While doing so, it is important to somehow indicate to the user that filters are active even if the menu is not currently open.