Radium

This is an emergency release for 5.9.15, where you in some situations could end up seeing a ton of error messages after clicking on a seqblock.

The release has also changed the usage of a couple of modifier keys:

1. To scroll horizontally when using the mouse wheel,
   you now have to press a Windows key (macOS: Alt),
   and not Shift. The reason is that Shift is used to
   limit moving the mouse pointer in the vertical direction,
   while the Windows (macOS: Alt) key is used to limit
   in the horizontal direction, so a Windows key seems
   more logical.
2. To quickly copy a seqblock, I've changed
   the keyboard modifier to Alt (macOS: Ctrl) instead
   of Shift. Then the Shift key can be used to limit moving
   seqblocks in the vertical direction.

Changes 5.9.15 -> 5.9.16:

• Updated the manual somewhat.
• Sequencer: Update graphics after undoing or redoing
  fade length.
• Instrument widget: Don't create undo entry for the Solo
  button unless undo for solo is enabled in preferences.
• Audio: Add option to create undo entries when clicking
  a solo button.
• Sequencer: Fix error messages popping up after just
  clicking a seqblock, without moving it.
• Sequencer: Make keyboard modifiers consistent by
  changing the modifier for copying seqblock from Shift
  to Alt (macosx: Ctrl). This way, Shift can be used to
  move a seqblock vertically.
• Mouse: Use the same keyboard modifier to scroll
  horizontally as used to limit mouse movement in the
  x direction. (Windows/Linux: windows key. macOS: Alt)
• Mixer/Manual/Faust: When using the mouse scroll wheel
  to scroll vertically, scroll to the left when
  scrolling the mouse wheel up, and to the right
  when scrolling the mouse wheel down. Not the other
  way around.
• Sequencer: Draw seqtrack number in a separeate box,
  not as part of the seqtrack name.
• Sequencer: Fix having to press mouse button two
  times to make mouse button work right after
  changing seqtrack name.
• Update s7
• API: ensureCleanStateOutsideMouseCycle