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_ShiftLeftForce.lua

@@ -0,0 +1,5 @@
+-- MIDI Grid: shift all notes one cell left even if some leave the item
+local dir = ({ reaper.get_action_context() })[2]:match("@?(.*[\\/])")
+local G = dofile(dir .. "midigrid_lib.lua")
+if not G.isActive() then return end
+G.runShift(-1, true)