new timer infrastructure

the usage of timers was reworked. Before this, every clock was derived
from a fast tick (the APRS NCO sample rate), which was sufficient until
now.
Three major things caused the rework: New clock source (16.367600MHz
TCXO), aprs baud rate not being able to have small enough error, exact
tuning requirement for DominoEX baud rate.

CCR0 is now used for the APRS NCO tick
CCR1 serves the APRS baud rate
CCR2 does RTTY baud rate
overflow does DominoEX baud rate (nice, because TCXO / 16 / 65536 =
coherent baud rate)

DCO is used when Si4060 is in SLEEP-mode, because it does not output the
DIVCLK then.
UART baud rate settings are adapted to the DCO or TCXO frequency. The
second timing is a little off with 8.0MHz, but this is negiglible.