Show delete button when long pressing a slide

[aforcier]

Closes #583. In addition to tapping the current slide to show the delete button, this PR also adds long press behavior. Long pressing any slide (no matter which slide you're currently on) will switch to that slide and show the delete button.

This build has been tested and 👍 'd by @megsfulton . She had some suggested improvements which we can make in the future - recorded those in #599 .

To test:

1. Create a story with multiple slides
2. Verify that long pressing the current slide shows the delete button and you can delete the slide
3. Verify that long pressing a slide you're not on switches to that slide and shows the delete button (and you can delete the slide)
4. Verify that long pressing the active slide again after step 2 or 3 hides the delete button
5. Verify that dragging to swap slides still works
6. Check that when slides are swapped, the delete button is hidden (since the long press action has been completed)

[mzorz]

Tested, works as expected, great job @aforcier !

LGTM