Config: Port EXI device and SI device settings to new config system.

[AdmiralCurtiss]

Separate PR here because of this snippet of code that I'm not sure how to translate correctly into the new system:

    for (unsigned int i = 0; i < SerialInterface::MAX_SI_CHANNELS; ++i)
    {
      int source;
      controls_section->Get(fmt::format("PadType{}", i), &source, -1);
      if (source >= SerialInterface::SIDEVICE_NONE && source < SerialInterface::SIDEVICE_COUNT)
      {
        StartUp.m_SIDevice[i] = static_cast<SerialInterface::SIDevices>(source);
        config_cache.bSetPads[i] = true;
      }
    }

This happens while parsing GameINIs and basically means 'if PadTypeN is set to a valid value, override SIDeviceN with that, otherwise keep the value set by SIDeviceN'. I can and have added these to GetINIToLocationMap(), but that's not quite the same behavior -- if you set it to an invalid value, Dolphin will now use SIDEVICE_NONE rather than the current SIDeviceN value. Are we fine with this? If not, how do we adjust the code so that previous behavior still applies?

[JosJuice]

I would be fine with this new PadType behavior.

[JosJuice]

There's a lot of casting going on in this PR. Is there any particular reason why the settings are defined as ints instead of the relevant enum type?

[AdmiralCurtiss]

I was somehow under the impression that you couldn't forward-declare an old C-style enum, but nope, seems to work fine. I've updated the settings to use the enum directly and removed the casts.

[leoetlino]

For the record, forward declaring an enum is fine as long as you specify the underlying type (which is only possible starting from C++11). Forward declaring an enum class without specifying the underlying type is okay too (int is the default).

[dolphin-emu-bot]

FifoCI detected that this change impacts graphical rendering. Here are the behavior differences detected by the system:

• aeon-charge-attack on mvk-osx-m1: diff
• burnout2-vehicletextures on mvk-osx-m1: diff
• DKCR-Char on mvk-osx-m1: diff
• DKCR-fast-depth on mvk-osx-m1: diff
• ea-pink on mvk-osx-m1: diff
• pm-hc-jp on mvk-osx-m1: diff
• rs2-skybox on mvk-osx-m1: diff
• spyro-bloom on mvk-osx-m1: diff
• ssbb-mod-lloyd on mvk-osx-m1: diff
• thps4-shadow on mvk-osx-m1: diff

_{^{automated-fifoci-reporter}}