Add Arduino GFX Library driver.

[goddade]

No description provided.

[ImpulseAdventure]

Hey @goddade -- thank you so much for this contribution!
As referenced in #532 I was not previously aware of Arduino-GFX, and it looks to be well supported. I like its ability to offer a wide range of display configurations.

Unfortunately I had overlooked the followup on #532 whilst being very busy at work. I suspect you may have already moved on, but if you (or others) still have an interest in having added support for Arduino-GFX into GUIslice, please post a reply to this PR and I'll look into it seriously.

My thought would be to revisit my idea of integrating the deltas that would be necessary in GUIslice_drv_adagfx as opposed to adding a whole extra duplicate for Arduino-GFX.

thanks!

[goddade]

Should I close this PR first and then put the changes into GUIslice_drv_adagfx?

[goddade]

I'm not working on this project for now, so it may take some time.