add support for multiplex signal parsing and syntax highlighting

[n0rbury]

Implements #4

Before:
Syntax highlighting for multiplex signals is buggy. Signals (and messages after that) don't show up in preview window.

After:

Attached is the sample dbc:

VERSION ""

NS_ :

BS_:

BU_:

BO_ 100 VehicleStatus: 8 Vector__XXX
  SG_ StatusCode M : 0|8@1+ (1,0) [0|255] "" ECU1
  SG_ Speed m0 : 8|16@1+ (0.1,0) [0|6553.5] "km/h" ECU1
  SG_ RPM m0 : 24|16@1+ (1,0) [0|65535] "rpm" ECU1
  SG_ Temperature m1 : 8|8@1+ (1,-40) [-40|215] "°C" ECU1
  SG_ Pressure m1 : 16|8@1+ (0.1,0) [0|25.5] "kPa" ECU1
  SG_ FuelLevel m2 : 8|8@1+ (0.5,0) [0|127.5] "L" ECU1

BO_ 200 SensorData: 8 Vector__XXX
  SG_ SensorID : 0|16@1+ (1,0) [0|65535] "" ECU2
  SG_ SensorValue : 16|32@1+ (0.001,0) [0|4294967.295] "V" ECU2