Implement the ADB keyboard

[mihaip]

Besides generating KeyboardEvents in the SDL event handler and returning the key state in the register 0 reads of the AdbKeyboard device, we also needed to generalize the ADB bus polling a bit. We now check all devices that have the service request bit set, instead of hardcoding the mouse.

The SDL key event -> ADB raw key code mapping is based on BasiliskII/ SheepShaver's, but cleaned up a bit.

To test this, I've been using the PDM ROM with a slightly tweaked 7.1.2 Disk Tools image that has the Key Caps DA installed.

[maximumspatium]

Please convert all those kVK_ANSI_x comments into an enum and use the corresponding constants instead of magic numbers.
I find the kVK_ANSI prefix misleading and propose to use kVK_x instead:

enum {
    kVK_A = 0x00,
    // ...
};

switch (event.keysym.sym) {
     case SDLK_a:            return kVK_A;
}

if (ke.key == kVK_CapsLock) {
    ke.flags = SDL_GetModState() & KMOD_CAPS ?
        KEYBOARD_EVENT_DOWN : KEYBOARD_EVENT_UP;
}

The definitions for the virtual keys should go into adbkeyboard.h.

[mihaip]

Please convert all those kVK_ANSI_x comments into an enum and use the corresponding constants instead of magic numbers. I find the kVK_ANSI prefix misleading and propose to use kVK_x instead.

Makes sense. I went with an AbbKey prefix for the enums, since kVK_ didn't seem quite right either -- they're raw ADB key codes, not virtual ones.