MAVExplorer: Add message descriptions to stats command

[shancock884]

The idea of this PR is to add the message description to the 'stats' command in MAVExplorer, using the downloaded XML metadata.
When I was first getting the hang of looking at logged data, I found myself using the stats command as a good to way to have an index of what messages exist, but the 4-letter names can take a while to get the hang of, so I thought that something like this would have made it easier!

Output is like this:

MODE  0.00%  [vehicle control mode information]
ARM   0.00%  [Arming status changes]
EV    0.00%  [Specifically coded event messages]
AUXF  0.00%  [Auxiliary function invocation information]
VER   0.00%  [Ardupilot version]
LAND  0.00%  [Slope Landing data]
DSF   0.03%  [Onboard logging statistics]
TERR  0.03%  [Terrain database information]
FMTU  0.03%  [Message defining units and multipliers used for fields of oth...]
RAD   0.03%  [Telemetry radio statistics]
ISBH  0.04%
MAV   0.06%  [GCS MAVLink link statistics]
...

As you see with FMTU, I have chosen to abbreviate long descriptions, to avoid making the actual stats bit any less readable.

[peterbarker]

Merged, thanks!