Add user friendly setWindowxxx(..) methods

[Stephane-D]

The current window setup methods :

void VDP_setWindowHPos(u16 right, u16 pos);
void VDP_setWindowVPos(u16 down, u16 pos);

are close to internals registers setup but not really user friendly.

Adding methods as :

VDP_setwnidowOff()
VDP_setWindowOnTop/Bottom/Left/Right(..)

would be much more easier to deal with.

Usage examples :
want a 32px high HUD at the top? (think shmups)
VDP_setWindowOnTop(4);

96px HUD at the side (don't forget width is in 16px steps, not 8px like height):
VDP_setWindowOnRight(6);

Set up window for vertical split screen (i.e. the right half of the screen):
VDP_setWindowOnRight(10);

[etherton]

Perhaps it makes sense to have the helper function units in pixels and >>3 or >>4 appropriately?