fix compilation error and minor runtime errors

First-time contributor, hopefully I am doing this right...

Fixed a problem with constructor reference semantics in the mne_browse_raw_qt application.  C++ does not allow a temporary to be bound to a non-const reference.  Since default arguments are temporaries, this was causing compilation errors in several classes where the constructor was declared with a default value for a reference argument.  Since C++ does allow binding temporaries to a const reference, I changed the argument type in these constructors to a const reference when possible.  The AverageWindow constructor takes a QFile that is mutable, so I had to overload the AverageWindow constructor.

Also noticed and fixed two runtime bugs in the mne_browse_raw_qt application.