Refactored the mission message exclusion logic.
The existing logic for filtering out MISSION* messages that don't apply to this instance of QGC wasn't clear and exactly correct. This refactoring reorganizes the code to be more clear and makes it explicit when the messages should be ignored by this instance of QGC.
Additionally a few more status messages are translated to be helpful to our non-English speaking devs/users.
Refactored mission manager code for handling MAVLink messages.
Also added translation calls for some UI messages.
Merge pull request #644 from Susurrus/mission_protocol

Mission protocol refactoring