QGCMessageView: Extensive code cleanup and bug fixing [DO NOT MERGE]

This is a follow up on #545. I've revamped the QGCMessageView to work like it should've:
Right-click menu works cleaner.
Coloring of output text is correct.
Output text now handles all severities of messages.

Still need to implement timestamping of messages, so don't merge yet! This is merely for feedback.
![message_view](https://f.cloud.github.com/assets/206363/2224068/219eeaec-9a7a-11e3-9953-df943753c42e.PNG)