audio/qtractor: update 0_9_37 → 0_9_38

Change-log:

• Track Properties (dialog): auto-background color option added.
• Mitigate undifferentiated disablement of up/down arrow buttons, on tempo and time-signature entry spin-boxes.
• MIDI clip editors (aka. piano-roll) are now raised and centered to current mouse pointer position in main timeline (tracks-view).
• Remember last Marker color setting in Tempo Map/Markers dialog.
• A colored ribbon is now featured on tracks-list Bus column and mixer-strip headers; ribbon colors are designated for Audio and MIDI tracks or buses, given by the respective '10dB' and 'Over' meter colors.
• Buses'In'/'Out' suffixes are back on mixer-strip header tooltips.