* Add a common/pimoroni.hpp to list default pins for various add-ons
* Move the BG SPI Slot enum here for safe keeping
* Switch all GPIO pin references to "uint" to match Pico SDK and bring back PIN_UNUSED as UINT_MAX
A new constructor has been added which accepts a enum type BG_SPI_SLOT which can be either:
ST7789::BG_SPI_FRONT or ST7789::BG_SPI_BACK.
This selects the correct CS/BL pins for the respective breakout-garden SPI slot.
Additionally the PWM'd backlight is turned *on* by default to avoid any confusion when the user does not set a value.
Phil Howard