Partial fix for 3477 (#3568)

Partial, as it will safe the state when suspending the reader by pressing the power-button.

However, it will still not safe the state when Pocketbooks autoPowerOff-Feature kicks in, as this does not create an event (e.g. no EVT_BACKGROUND) of any kind.

* Clean up device.lua

* Save state on suspend

frontend/ui/uimanager.lua

@@ -74,6 +74,16 @@ function UIManager:init()
             Device:reboot()
         end)
     end
+    if Device:isPocketBook() then
+        self.event_handlers["Suspend"] = function()
+            self:_beforeSuspend()
+            Device:onPowerEvent("Power")
+        end
+        self.event_handlers["Resume"] = function()
+            Device:onPowerEvent("Power")
+            self:_afterResume()
+        end
+    end
     if Device:isKobo() then
         -- We do not want auto suspend procedure to waste battery during
         -- suspend. So let's unschedule it when suspending, and restart it after