Make member variable underscores a suffix instead of a prefix

This change is made because tokens prefixed with underscore are reserved.
Renamed C++ headers to .hpp
Upgraded cotire to 1.6.6
MainWindow: Include device/file name in window title

  This fixes bug #244
SamplingBar: Reduced indentation in update_sample_count_selector
Check LIMIT_SAMPLES and SAMPLERATE are available to read before attempting to read them