The SongsView for a large Playlist takes far too long to load

[Victor Thompson]

A playlist that has a modest number of songs (50) doesn't take long to load, but once this exceeds 100 or so it takes exponentially long to load the playlist song count in the SongsView. The UI is mostly locked up the whole time the playlist loads, as well.

To reproduce:

1. Navigate to the Songs tab
2. Long press any item to enter multiselect mode
3. Tap the checkbox icon in the header to select all
4. Select the overflow icon in the head, then "Add to playlist"
5. Create a new playlist or use an existing one and select it.
6. Navigate to the Playlists tab
7. If the playlist created or used in Step 5 has under 100 songs, repeat from Step 1 until it does.
8. Tap the playlist created or used in Step 5

Actual results: The page/view selected in step #8 is delayed in launching and takes excessive time to load blocking the UI while it does so.

Expected results: It loads without delay.

[Ubuntu Phone Apps Jenkins Bot (ubuntu-phone-apps-jenkins-bot)]

Fix committed into lp:music-app at revision 901, scheduled for release in music-app, milestone 3.0