Mavlink FTP implementation List command

List commands are now working fully. This includes handing things like Ack timeouts and Naks correctly. Added new support to MockUAS which allows for mavlink mock plugins o be created. Created a mavlink ftp server plugin which is used to create the unit test QGCUASFileManager.

After this gets merged. My next pull will be file download implementation.

Note: Requires use of the PX4Firmware:mavlink-ftp branch to run against a Pixhawk board. Also, there appears to be a bug in PX4Firmware which causes Acks to be lost at times. I've got an email thread going with Lorenz on that. This will likely require a pull on the firmware side as well to make this all work reliably.