Add battery tooltip support

This adds a new config option 'battery_tooltip' (enabled
by default), which can be used to enable a tooltip for
the battery widget providing details for all installed
batteries.

src/config.c

@@ -434,6 +434,11 @@ void add_entry (char *key, char *value)
 			percentage_hide = 101;
 #endif
 	}
+	else if (strcmp (key, "battery_tooltip") == 0) {
+#ifdef ENABLE_BATTERY
+		battery_tooltip_enabled = atoi(value);
+#endif
+	}
 
 	/* Clock */
 	else if (strcmp (key, "time1_format") == 0) {