Draft: initial implementation of playlist selection

Reason for the change

Adds a selection mode for the playlist view which is useful when removing multiple items at once. In the future I would like to add a feature where you can select an entire album at once by clicking a checkbox in the album header item.

Screenshots

Current appearance:

Test plan

• Need to improve autotests
• Needs more manual testing

Manual tests that I have tried so far:

• Select some items, then shuffle the playlist. The same items are still selected.
• Select some items, then move them around in the playlist. The same items are still selected.
• Select some items in shuffle mode, then add multiple tracks. Ensure that tracks are added before and after the selected items. The same items are still selected.
• Select some items them remove items before and after the selected items. The same items are still selected.
• Select some items, then remove only one of the selected items (using the hover toolbar). The other items are still selected, no new items are selected.
• After each of the above steps, remove the items from the playlist. Only the items that were selected were removed.

Screenshots or screen recordings