refactor refresh

Lots of the device-related distinction wandered into
base/ffi/framebuffer_<driver>. This eases the refresh logic in
UI manager, which basically only decides what kind of refresh
to trigger. The device specific configuration in the framebuffer
driver decides how to realize that whish.

screen.lua is gone, in its place is now the framebuffer driver.
The device abstraction decides what framebuffer driver to load.