Add whole-take time shifting

Ctrl+Shift+Left/Right move every note in the take by one grid cell, for
clips that landed off the beat.

Since the whole take moves together, notes cannot collide with each other;
the hazard is the item boundary. Notes pushed past either end stop sounding
but are not deleted, so the operation is reversible -- shifting back restores
them. The plain keys still refuse when that would happen and report the count,
because a clip that has silently lost its first bar is a bad thing to discover
later. Ctrl+Alt+Shift forces it and names the consequence.

Positions are converted through QN rather than offset in raw ticks, so shifts
stay musically correct across tempo changes.

midigrid_lib.lua

@@ -352,6 +352,83 @@ function M.toggleCell(take, pitch)
   return result, detail
 end
 
+---------------------------------------------------------------- time shift
+
+--[[
+  Move every note in the take by whole grid cells.
+
+  Because the entire take moves together, notes cannot collide with each
+  other -- the only thing at stake is the item's boundaries. Notes pushed
+  outside them are not deleted, they simply stop sounding, and shifting back
+  the other way restores them. The safe variant still refuses, because
+  silently silencing part of a clip is exactly the kind of thing you want to
+  be told about rather than discover later.
+
+  Positions are converted through QN rather than offset in ticks, so a shift
+  stays musically correct across tempo changes.
+
+  Returns moved, outside. moved is nil when the shift was refused.
+]]
+function M.shiftAllNotes(take, cells, force)
+  local _, notecnt = reaper.MIDI_CountEvts(take)
+  if notecnt == 0 then return 0, 0 end
+
+  local g = M.gridQN(take)
+  local item = reaper.GetMediaItemTake_Item(take)
+  local itemPos = reaper.GetMediaItemInfo_Value(item, "D_POSITION")
+  local itemLen = reaper.GetMediaItemInfo_Value(item, "D_LENGTH")
+  local loPPQ = reaper.MIDI_GetPPQPosFromProjTime(take, itemPos)
+  local hiPPQ = reaper.MIDI_GetPPQPosFromProjTime(take, itemPos + itemLen)
+
+  local moved, outside = {}, 0
+  for i = 0, notecnt - 1 do
+    local ok, _, _, s, e = reaper.MIDI_GetNote(take, i)
+    if ok then
+      local ns = reaper.MIDI_GetPPQPosFromProjQN(take,
+        reaper.MIDI_GetProjQNFromPPQPos(take, s) + cells * g)
+      local ne = reaper.MIDI_GetPPQPosFromProjQN(take,
+        reaper.MIDI_GetProjQNFromPPQPos(take, e) + cells * g)
+      moved[i] = { ns, ne }
+      if ns < loPPQ - SLACK or ne > hiPPQ + SLACK then outside = outside + 1 end
+    end
+  end
+
+  if outside > 0 and not force then return nil, outside end
+
+  reaper.Undo_BeginBlock2(0)
+  for i = 0, notecnt - 1 do
+    local np = moved[i]
+    if np then
+      reaper.MIDI_SetNote(take, i, nil, nil, np[1], np[2], nil, nil, nil, true)
+    end
+  end
+  reaper.MIDI_Sort(take)
+  reaper.Undo_EndBlock2(0, "MIDI Grid: shift notes", -1)
+
+  return notecnt, outside
+end
+
+-- Shared driver for the four shift actions: does the work and the speech.
+function M.runShift(cells, force)
+  local hwnd, take, err = M.editor()
+  if not take then M.say(err) return end
+
+  local moved, outside = M.shiftAllNotes(take, cells, force)
+  local dir = cells < 0 and "left" or "right"
+
+  if moved == nil then
+    M.say(("Not shifted %s, %d %s would fall outside the item. Add Alt to force.")
+      :format(dir, outside, outside == 1 and "note" or "notes"))
+  elseif moved == 0 then
+    M.say("No notes to shift")
+  elseif outside > 0 then
+    M.say(("Shifted %s, %d %s now outside the item and silent")
+      :format(dir, outside, outside == 1 and "note" or "notes"))
+  else
+    M.say(("Shifted %s, %d %s"):format(dir, moved, moved == 1 and "note" or "notes"))
+  end
+end
+
 --------------------------------------------------------------- note length
 
 --[[