Preload recents on settings change or dpi change

Fix glitches on settings change like app icons size not being refreshed
after changing the panel size if the user shows the panel before the icons
background thread is still doing its job
This will also give a smoother panel on the first toggleRecents event
e.g. after changing the icon pack

TODO: preload stuff also after the first boot, adding a preload call
when creating the controller atm can give unwanted behaviors (like empty
panel after boot at the first toggle event)

Change-Id: I22ea14d728ab71e4d9cc590e4ab9d3979708b654