Remove default use of 16781904 as value 0

This lets the user assign the value themselves, which may work out
better for them since non-English keyboard layouts put different glyphs
to the left of the number 1 key and we cannot account for all possible
keyboard layouts.

src/prompter/Prompter.qml

@@ -214,7 +214,7 @@ Flickable {
         property int markerToggleModifiers: Qt.ControlModifier
         property int namedMarkerToggle: Qt.Key_M
         property int namedMarkerToggleModifiers: Qt.ControlModifier | Qt.ShiftModifier
-        property int setVelocity0: 16781904  // Key: `
+        property int setVelocity0: 0
         property int setVelocity0Modifiers: Qt.NoModifier
         property int setVelocity1: Qt.Key_1
         property int setVelocity1Modifiers: Qt.NoModifier