feat: map SDL scancodes rather than keycodes

[giuliof]

This way, the user's keyboard layout doesn't affect how actual keys are mapped inside the emulator.

[giuliof]

Obv, since the SANCO keyboard is not an ISO/ANSI compliant one, this PR would need some changes on CP/M-side keymap, i.e. SLF80037_US.TXT (rename it to .COM, github don't allow me to upload this file). But this is the same one used on the physical machine to have an usable layout. Original SLF file would produce a french(ish) keyboard layout.

[giomba]

I report here what we discussed on the phone.

• since SDL2 allows us to handle different keyboard layouts using scancodes, we should use scancodes instead of keycodes
• then, we should pass the ISO/ANSI-like scancodes from the user's keyboard, to the emulator logic, and to the emulator software
• the Sanco's CP/M can already handle different keyboard layouts (using different SLF800* autoexec-like executable)
• we can then modify the unofficial CP/M disk image in order to allow the user to select its preferred keyboard layout
• we want to support the following keyboard layouts: US English (because it's widespread and well-known), IT (because me and other Italian contributors use it), and obviously FR (because this actually is a French computer)
  Linked CP/M issue: Modify SLF800* in order to support different keyboard layouts ceda-cpm#2

[giomba]

I added a commit on top of your in order to support the extra key in ISO keyboards, which is iseful for European layouts.