Add tlf keyboard mappings

[m5evt]

tlf keyboard mappings are sensible and already known to linux contesters (?). Implement the following:

• Page Up/Dwn for CW speed
• left/right arrow to change band
• plus key to toggle S&P/run mode
• minus deletes last qso

[m5evt]

Don't delete last QSO, simply mark it with an 'X'?