Slideshow button enhancements

[dan-bowerman]

Description

From @AleksueiR's UX review regarding dynamic slides from last summer:

Right now, the buttons that let you go back and forth in the slideshow jump up and down when the slides are different sizes, and that's confusing.

Is your feature request related to a problem?

Consistency and expected behaviour of galleries.

Proposed solution (also from @AleksueiR's UX review)

• The button height should match the height of the slideshow component with the icon vertically centered, so even though the icon will move up or down when switching between slides of different height, you can still click the next/previous button without moving the cursor.
• Add a hover effect on the back/forward buttons so the users knows they can still click them even though the button label had moved.
• It would be cool if the slideshow animated changes in its height as you go through it. This could make the changes between slides look smoother and remove the annoying jump of the next/previous button icons.>

Additional context

#353
ramp4-pcar4/storylines-editor#225