Resolves issue with AFC and frequency change event handling,

specifically for P25 C4FM frequency tracking.

-Clarifies frequency change event naming and adds request/notitification
events for channel frequency correction type events

-Removes DC offset filter from P25 C4FM demodulator which was operating
independently of the AFC tracking component within the C4FM symbol
filter.

-Changed FCD tuner center DC spike buffer size after detecting that the
DC center spike was influencing the P25 C4FM filter frequency tracker
whenever the tuner's center frequency was changed to bring the channel
boundary within 3 kHz of the center spike -- set it to 5 kHz to provide
a better buffer.

-Updated spectral channel display to provide better indication of
automatic frequency control (yellow line) versus channel frequency

src/module/decode/DecoderFactory.java

@@ -256,7 +256,7 @@ public class DecoderFactory
 				switch( modulation )
 				{
 					case C4FM:
-						modules.add( getFMDemodulator( decodeConfig, 6750, 7500, REMOVE_DC ) );
+						modules.add( getFMDemodulator( decodeConfig, 6750, 7500, NO_REMOVE_DC ) );
 						modules.add( new P25_C4FMDecoder( aliasList ) );
 						modules.add( new P25DecoderState( aliasList, channelType, 
 								Modulation.C4FM, p25Config.getIgnoreDataCalls() ) );