Update ImageWidget to support more stretch settings, and enable screensaver on kobo

reader.lua

@@ -97,7 +97,12 @@ local UIManager = require("ui/uimanager")
 local Device = require("device")
 local Font = require("ui/font")
 
--- read some global reader setting here:
+-- change some global default values according to the device types
+if Device:isKobo() then
+    KOBO_SCREEN_SAVER = "/mnt/onboard/screensaver"
+end
+
+-- read some global reader settings here:
 -- font
 local fontmap = G_reader_settings:readSetting("fontmap")
 if fontmap ~= nil then