rtmp-stream: Add statistics callbacks

Add callbacks for total bytes sent and number of dropped frames
Add function to main window to query CPU usage
Implement status bar

The status bar now displays:
 - Auto-reconnect information (reconnecting and reconnect success)
 - Dropped frames (as well as percentage of total video frames)
 - Duration of session
 - CPU usage of the program
 - Kbp/s

The OBSBasic class is getting a bit big, so I separated out the
status bar code to its own class derived from QStatusBar.