Fix size of icons in menus inside apps when shown as images

Some popover menus, like the contacts menu, still show their icon using
an img element. The main CSS rules assume that a "content-box" sizing is
being used, and thus set the size and padding of the image to add up to
the line height.

However, ".app-*" descendants use a "border-box" sizing, so when a menu
with an image was shown in an app the icon was not properly shown. Now
both the width and height of the image is set to the item height in
those cases, which causes the visible size of the icon to be the item
height minus the padding (the same as when "content-box" sizing is
used).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>