panel: Indicate focus using a pill-shaped background

panel: Indicate focus using a pill-shaped background

The current way of indicating focus of elements in the panel does not
work very well with a fully-transparent panel, a line at the bottom of
the panel doesn't make too much sense if there is no real panel, but
only the text and icons.

To make the indicators look better in this case, switch to a pill-shaped
background color to indicate the focus of items in the panel.

For this to look good, there has to be a small black border above and
below the background, this also requires increasing the height of the
panel (from 1.86em to 2.2em) for visual purposes.

Also, since we now no longer need to color the lower bottom of the
panel, we can remove the custom drawing code for the border of the
panels corner, so do that.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1397>