power: Make sure we fall back to the correct icon
Commit bd18313d changed to a new naming scheme for battery icons, and used to old icon names as fallback-icon-name for compatibility with older/other icon themes.
However that fallback code isn't working correctly, as GThemedIcon's
default fallbacks will transform a name of battery-level-90-symbolic
to a list of names:
battery-level-90-symbolic
battery-level-symbolic
battery-symbolic
The last one frequently exists, so instead of the intended fallback, we end up with a generic battery icon.
Address this by specifying the icon as GIcon instead of an icon-name, where we have more control over how the icon is resolved.
Fixes #1442 (closed)