ssd: support window icons

ssd: support window icons

The default `titleLayout` is updated to `icon:iconify,max,close` which
replaces the window menu button with the window icon.

When the icon file is not found or could not be loaded, the window menu
icon as before is shown.

The icon theme can be selected with `<theme><icon>`.

This commit adds libsfdo as an optional dependency. `-Dicon=disabled` can
be passsed to `meson setup` command in order to disable window icon, in
which case the window icon is always replaced with a window menu button.