Improve support for 8bitdo SNES controllers

[glutanimate]

This issue report tracks the progress in adding full compatibility for 8bitdo SNES controllers.

Current issues:

• 8bitdo controllers map the D-Pad to the x- and y-axis, so there is no way to assign the dpad to anything other than the current axis actions (should be addressed by #3)
• because the 8-bitdo D-Pad is not pressure sensitive, the gamepad always receives maximum deviation events, which makes scrolling too fast. Adding axis sensitivity as in #2 should fix that

Debug information

Sample layout of an 8bitdo SNES bluetooth controller (thanks to Omar for his help in elucidating this info!):

[danielhigham]

Here's the debug info for an 8bitdo SN30 bluetooth controller (not the 'pro' version which has two analog sticks):

===ANKI=== Anki 2.1.14 (7b93e985) Python 3.6.7 Qt 5.12.1 PyQt 5.11.3 Platform: Mac 10.14.6 Flags: frz=True ao=True sv=2 Add-ons: Image Occlusion Enhanced for Anki 21 alpha Mini Format Pack Frozen Fields True Retention Edit LaTeX build process AnJoy bettertags Cloze Overlapper ===ANJOY=== ```json version: 0.1.0-beta.1 [ { "id": "Wireless Controller (STANDARD GAMEPAD Vendor: 054c Product: 05c4)", "index": 0, "connected": true, "timestamp": 59695923.800000004, "mapping": "standard", "axes": [ -0.003921568393707275, -0.003921568393707275, -0.003921568393707275, -0.003921568393707275 ], "buttons": [ { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 }, { "pressed": false, "value": 0 } ], "vibrationActuator": { "type": "dual-rumble", "playEffect": "function playEffect() { [native code] }", "reset": "function reset() { [native code] }" } }, null, null, null, 4, "function item() { [native code] }" ] Index: 0 ```

[glutanimate]

Thanks, Daniel! This should be very helpful.

It seems like GitHub isn't collapsing longer comments automatically, so I took the liberty to use some formatting tricks on the log. Hope you don't mind!