Atari800,Atari5200

A lot of changes from Wojciech Mostowski:

- Most of the currently reported bugs referring to basic functionality (like Demo X or Y not working, reloading carts requiring core reboot, etc.) are fixed. To the best of my knowledge, there are currently no known issues in this regard, at the bottom I included the list of issues from the tracker that are for sure or should be fixed,

- A direct, DMA based fast (SIDE3 / AVG speed class :)) XEX loader,

- Substantially extended range of supported cartridge types, including DCART, and also Super Cart for the 5200 core allowing to run many of the new homebrew titles on 5200,

- An option to boot the D1: disk directly (for easy gaming and the lazy ones not wanting to press Option and Reset),

- An option (triggered by OS selection, explained in the README file) to work with the pure 800 mode and its common memory configurations, including a 4 controller support,

- The 5200 core now also supports 4 controllers (and now has joystick 1 and 2 swap option that can be useful),

- Fixed and fully working ATX support with the ability to choose between 1050 and 810 (timing) behaviors, all ATX tests by DjayBee of AtariAge are reporting expected results.

- Improved Joy2B+ support (this was more about the extreme pot positions that need to be at a certain value),

- Addition of D3 and D4 drives,

- Ability to stack carts on top of a stacking-enabled SDX cart, alternatively mount left and right carts separately in 800 mode,

- Special support to load Ultimate 1MB and SIDE2/SuperCart SDX ROM images including Ultimate RT clock emulation (fully working with initialization from the MiSTer system clock),

- Many core UI improvements, including an option to clip the sides of the screen to hide GTIA garbage, structured menus, and loading / setting up OS images from the UI,

- A PBI ROM emulation with a simple Atari BIOS that enables direct DMA communication with the mounted ATR images, alternatively provides the HSIO implementation, so functionally the same as what AVG/SUBCart or SIDE3 are doing, all this configurable from the MiSTer menu. Additionally it supports HDD raw images (*.img) and implements the Atari Partition Table API to work with SDX disk utils. The ins and outs of this are described in the README file.

- An improved sound mixer for Pokey / GTIA sound / Covox that is more accurate (non-linearity scaling from the recent PokeyMax implementation by Mark) and prevents sound level jumping in some corner case situations (most audiable in Rewind 2 demo somewhere after the 2nd minute),

- Pokey serial timing fix making all ACID tests now pass with the core,

- The external SIO support has been tested with the A8-Pico-SIO device, a fix for a small bug identified is provided (reversed motor line signal),

- The 4MB memory option has been re-implemented to follow the Axlon banking method (also for the pure 800 mode), getting rid of the non-functional PORTB based banking (it is actually known not to work well on the original hardware).