ui: Add borders and padding to installed and search lists

This makes them look more like the mockups:

• https://gitlab.gnome.org/Teams/Design/software-mockups/-/blob/master/installed-view.png
• https://gitlab.gnome.org/Teams/Design/software-mockups/-/blob/master/search-results.png

Signed-off-by: Philip Withnall pwithnall@endlessos.org

Fixes: #914 (closed)