Megadropdown animation on desktop

[gfellerph]

The megadropdown currently has no animation on desktop. Add a slide-out animation similar to what's currently on https://post.ch.

Behaviour on desktop

1. Megadropdown trigger is clicked -> Megadropdown slides out
2. Trigger, close button or ESC are pressed -> Megadropdown slides up

Behavior when switching between menus

1. Megadropdown trigger is clicked -> Megadropdown slides out
2. Another trigger button is clicked -> Megadropdown content changes without animation
3. Currently active trigger, close button or ESC are pressed -> Megadropdown slides up