#2 Memory consumption

[Stefan Bormann]

LoconetMonitor never frees memory for displayed messages.
If LoconetMonitor keeps running with a lot of traffic, it eventually eats up quite a lot of memory.
Proposed fix: set a maximum number of displayed and memorized messages. Older messages are deleted.
TBD: The maximum number could be a constant or configuration.
TBD: Does it make sense to switch this off to have the maximum possible history?