Add support for switches which aren't part of the original switch matrix

[mkalkbrenner]

• direct flipper buttons
• coin door switches directly wired to the CPU
• custom buttons

Those buttons should be treated like switch matrix buttons regarding the protocol so that all features like fastflips are supported without changes. libppuc should simply forward them like switch matrix buttons.

ppuc/pinmame must treat them differently. Some need to be send to the CPU others should be ignored.

We should follow the pattern of VPX. That's the mapping for Williams System 6:

'-------------------------
' S6 Data
'-------------------------
' Flipper Solenoid
Const GameOnSolenoid = 23
' Cabinet switches
Const swAdvance      = -7
Const swUpDown       = -6
Const swCPUDiag      = -5
Const swSoundDiag    = -4
Const swMasterEnter  = -3
Const swTilt         =  1
Const swBallRollTilt =  2
Const swStartButton  =  3
Const swCoin3        =  4
Const swCoin2        =  5
Const swCoin1        =  6
Const swSlamTilt     =  7
Const swHiScoreReset =  8
Const swLRFlip       = 82
Const swLLFlip       = 84
Const swURFlip       = 81
Const swULFlip       = 83

1-8 are part of the matrix, 81-84 are "virtual" (they directly power the flippers, no connection to the CPU).
The negative numbers are the switches directly wired to the CPU.

https://github.com/vpinball/vpinball/blob/a491317195b1f686eec7f30a23111d0e5c8a3c4b/scripts/s6.vbs#L17-L39

[mkalkbrenner]

SwitchState already supports negative numbers:
https://github.com/PPUC/libppuc/blob/ad4af67c58dd214e34a630b4503d1b751be50b38/src/PPUC.h#L31

libpinmame accepts these:
https://github.com/PPUC/ppuc/blob/50dee359261c5d4805e1436041348060759f7592/src/ppuc.cpp#L630

Our event IDs are uint8_t. So we need to define some custom IDs and convert them into negative numbers here:
https://github.com/PPUC/libppuc/blob/ad4af67c58dd214e34a630b4503d1b751be50b38/src/RS485Comm.cpp#L360

[mkalkbrenner]

Let's map IDs above 240 to negative values:

241 => -1
242 => -2
...
247 => -7
...
254 => -14
255 is reserved.

Switch numbers between 200 and 240 could be custom switches.