accelerometer event icons are confusing

[tballmsft]

The icons (tilt-up/down, etc.) are confusing - need to make sure the icons align with the actual events. Also, the names are confusing. The current events:

• "shake", all good
• "tilt up", really is "bottom edge towards earth gravity"
• "tilt down", really is "top edge towards earth gravity"
• "tilt left", really is "left edge towards earth gravity"
• "tilt right", really is "right edge towards earth gravity"

We are missing

• "face up", microbit front side towards earth gravity"
• "face down", micro:bit back side towards earth gravity"

[tballmsft]

Added "face up" and "face down": https://github.com/microsoft/microcode/pull/500

[tballmsft]

@pelikhan notes that the events were are using are from the micro:bit, where the micro:bit is assumed to be lying flat on table, but with MicroCode that's not the case when the micro:bit is plugged into the shield (facing the user).