Multiple item selection

[CDrummond]

Update browse and queue pages to support multiple item selection. e.g.

- Long press on a playable item
  - change its image/icon to a check
  - change sub-toolbar to be
    [x] <selected count> (play now) (insert) (append)   -- for browse page
    [x] <selected count> (remove)  -- for queue page
  - clicking the [x] disables select mode
  - clicking buttons on left will apply action to selected items
- click any item to add to selection
- browsing is not possible until selection mode stopped

[CDrummond]

Implemented for browse and queue views. Multiple drag'n'drop of queue still todo.