Defer searches until all albums have been imported 

When importing an artist (e.g. from search), this process will fail when the album search fails. To be more consistent: defer searches until all requested albums are imported, then start searching. If search now fails with some exception, then the album import is already finished.

P.s. there is a search exception in `develop` branch right now, when searching for extras is enabled. I am working on a big pull request to optimize a lot of code, which should fix the exception. Check [this branch](https://github.com/basilfx/headphones/tree/requests) out for current status, but it requires some testing before I will send a pull request.