Rework charset selection

[okarmazin]

Different printer manufacturers assign the same code page numbers to different charsets.

Printer P1 might assign Code Page 50 to IBM800, while another printer P2 might assign Code Page 50 to Windows1250.

Currently charsets have an assigned code page number which is taken from Epson's ESC/POS documentation. When a charset is selected in CommandBuilder, the hardcoded code page number is sent to the printer. In order to work across different manufacturers the code page number has to be decoupled form the charset.

Supersedes #35