Add support for mapping analogs to D-pad again.

Use a cleaner approach which can feasibly work on all platforms.
Uses input_push/pop_analod_dpad to modify the joyaxis field before
polling input. This way, the RetroPad D-pad binds can inherit the
joyaxis fields from ANALOG_LEFT_X_PLUS and friends.

Also add support for the field in RGUI and saves settings.
Push analog dpad mode for autoconfig as well.
Merge pull request #481 from libretro/dpad-map

Analog D-pad mapping