Add generic context-sensitive joystick actions

[AliceLR]

Add generic joystick actions to allow joystick inputs to be more consistent across different interfaces and to hopefully make input mapping between consoles more consistent and useful.

• Use joystick helper functions in core.c
• Config support for joystick actions
• Joystick action autorepeat
• Joystick handler functions for contexts
• SDL: update primary joystick when a joystick is plugged/unplugged Allow all joysticks to send UI actions
• SDL: replace the Pandora joystick mapping hack with explicit checks in SDL_KEYUP/SDL_KEYDOWN so the proper joystick press/release functions can be used.
• Fix NDS port, update pad.config
• Fix 3DS port, update pad.config
• Fix Wii port, update pad.config
• Update PSP pad.config
• Update GP2X pad.config
• Add act_ prefixes to actions so they can't be confused with key names
• Automatically generate mappings for SDL 2.0 platforms
• Add more SDL mappings using SDL_GameControllerDB
• SDL mappings shouldn't overwrite user-defined config mappings
• Config options to change default SDL→MZX actions, to add user-defined SDL_GameController mappings, and to disable automated joystick mapping altogether
• pad.config files need better standardization
• General feedback/testing