This makes the NavBar toggle use radius_s for the button radius, to match designs. Adding a new enum value just for that feels a bit messy, but using Button::Custom feels potentially messier (and I'm not entirely sure how to do it). This could be removed if it's decided to use radius_s for all header bar buttons in the future.
Introduces a horizontal_padding field to the header bar, which is needed for the padding offset, but can also be used by cosmic-comp to make window controls more centered in the SSD corners. Though this can be removed if it'll be eventually possible to set the borders on individual container sides, which would remove the need for the offset, and if SSDs should have the same horizontal padding of 8 as apps. I removed this, since it caused the header elements to be off-center at the corners.

Also fixes #743.

Edit: Also makes the styling of the dropdown closer to designs.