upower: Add 'low' and 'critical' CSS classes

Add secondary CSS class based on the 'warning_level' field reported by UPower
over D-Bus.  This makes it possible to add custom styling when the battery is
near empty.

include/modules/upower/upower.hpp

@@ -71,6 +71,7 @@ class UPower : public AModule {
   GDBusConnection *login1_connection;
   std::unique_ptr<UPowerTooltip> upower_tooltip;
   std::string lastStatus;
+  const char *lastWarningLevel;
   bool showAltText;
   bool showIcon = true;
   bool upowerRunning;