OpenDingux: Implement button remapping saved to file.

More technically:
* Custom persistence functions for mappings to save them to a file in a format that makes sense (^ v < > L R A B X Y S s, with X and Y under the SNES/Nintendo DS/Dingoo A320 mapping, S for Start and s for Select).
* Merge the GCW Zero and Dingoo A320 input code, dealing with the face button placement issue (X and Y are reversed on one compared to the other) by having different descriptions #ifdef GCW_ZERO and #ifdef DINGOO_A320.
* Custom functions to nullify Left/Right, to display button remapping values and to get buttons for mapping.
* Display settings are now in their own submenu.
* There is now a way to get raw button presses in the GUI.