Move lvgl display init from main.cpp into LittleVgl.cpp to be closer to
InfiniTime, where display initialization is also done in LitteVgl.cpp.
Enable the original FlushDisplay code to get the screen
transition animations like on the real PineTime.
Also slow down the rendering, to actually be able to see the screen
flushing.
For the Up and Down screen transitions implement the screen movement.
When moving Down, move the the whole screen content down, and then draw
the new part of the new screen at the top of the display. Repeat until
screen transition is finished.
Fixes: https://github.com/InfiniTimeOrg/InfiniSim/issues/13
