Re-evaluate VKeyboard & autocmd design

[ColinPitrat]

Currently the virtual keyboard inject SDL events.

To do this:

• for regular char, it maps the char (from the button name) to a SDL event (SDLkeysFromChars)
• for special keys and emulator commands, it uses SDLkeysymFromCPCkeys on a CPC key

The drawback of this design are:

• it depends on the keyboard layout. We have to test it with all layouts (keymap_*.map) to be sure that it works fine
• some keys can be problematic (no ideal SDL event corresponding to the CPC_key) whereas the purpose of the virtual keyboard is exactly to provide a workaround for this

It should be possible to map the string to CPC keys and inject them instead. It requires:

• removing SDLkeysFromChars (it's only used for this) and replacing it by CPCkeysFromChars
• stop posting a KEYDOWN / KEYUP event and post a USEREVENT instead, or even not post an event and handle the key change (call applyKeyPress) directly. Problem: This doesn't cover emulator specific keys (MOD_EMU_KEY). The keyup/keydown logic processing the cpc_key (applyKeyPress + emulator specific) could be moved into a separate function that would be called both for KEYDOWN, KEYUP and USEREVENT
• User event code would be either DOWN or UP, data1 would be the cpc_key