PianoRoll: increase start key by one octave

When opening the Piano Roll for the first time and clicking on "A", you
would expect the same note that is previewed in the preview pane (A5),
but it is instead one octave lower (A4).

Fixed this by adusting INITIAL_START_KEY.

Closes #94.

src/gui/piano_roll.cpp

@@ -106,7 +106,7 @@ const int RESIZE_AREA_WIDTH = 4;
 const int NE_LINE_WIDTH = 3;
 
 // key where to start
-const int INITIAL_START_KEY = Key_C + Octave_3 * KeysPerOctave;
+const int INITIAL_START_KEY = Key_C + Octave_4 * KeysPerOctave;
 
 // number of each note to provide in quantization and note lengths
 const int NUM_EVEN_LENGTHS = 6;