In MAVLinkInspector, the messages are separeted by sysid and msgid

Hi!

In MAVLinkInspector widget, I have implemented the sorting of the message by sysid in addition to msgid (i.e. the messages are first sorted by sysid and inside the sysid by msgid. This is really useful when you have many UAS connected at the same time. 

Before, when many UAS were sending messages, the frequencies were added ( except if we start to select specific UAS) and therefore it was hard to say rapidly if a UAS was sending at a wrong rate or if many UAS were sending at a correct rate (see image). In addition, if UAS were not sending the same messages, it is hard to say what is correct, who is sending what and how fast. 
![qgc_sorted_only_by_msgid](https://cloud.githubusercontent.com/assets/6390059/4201690/731e13b2-3821-11e4-8d0f-905225b7c658.png)

Now, a tree is created for each UAS with their respective messages and rates. 
![qgc_sorted_by_sysid_n_msgid](https://cloud.githubusercontent.com/assets/6390059/4201756/eab9b2dc-3821-11e4-9baf-904d4d5dc805.png)

The final goal of such implementation will be to be able to switch on and off any stream from a particular UAS. 

TESTING:

Tested on OS: Win 7 SP1 x64, Linux 14.04 64 bits
Tested in flight: Yes (>10 flights)