Offset and resize Kobo Aura screen because of obscuration by black bezel

The visible screen of the Kobo Aura is smaller due to the black bezel that limits the visible area. This commit fixes this. The minibar is now inside the visible screen area.

`wtest.lua` is updated. Extra requires and paths are added. Also the function `TestVisible:paintTo(bb)` is updated to check the visibility and dpi.

In `uimanager.lua` the functions `offsetX()` and `offsetY()` are introduced to add an offset to `widget.x` and `widget.y`. The offsets for non-Kobo Aura devices remains zero.

In `screen.lua` the functions `getWidth()` and `getHeight` are changed to allow hardcoding the dimensions of the visible screen. Non-Kobo Aura devices are not affected.

Limitations:
 - Full screen refresh uses the new screen dimensions. Artifacts drawn outside the visible screen remain forever. These remainders are due to change of rotation mode.