Add play-from-start-of-bar

Alt+Space plays from the downbeat of the bar the cursor is in. Auditioning a
cell says what a note is; playing from the bar line says whether it is in the
right place, which is the question that matters at 16ths and 32nds and cannot
be answered from a note in isolation.

REAPER can almost do this by chaining a go-to-measure action with play, but
that leaves the edit cursor at the bar line -- and the edit cursor is the grid
cursor, so you would lose your editing position every time you listened. Here
the cursor is moved just long enough to start playback and then restored with
seekplay=false so the restore cannot drag playback with it.

Works regardless of grid mode, being a listening aid rather than an edit.

README.md

@@ -81,6 +81,7 @@ actions by hand in Actions → Show action list. That always works.
 | `Alt+Shift+Up` / `Alt+Shift+Down` | Velocity up / down | — |
 | `Alt+G` | **Toggle grid mode on/off** | — |
 | `Alt+H` | Toggle hold mode | — |
+| `Alt+Space` | Play from start of the current bar | works in both modes |
 | `Alt+A` | Repeat current cell (speak + sound) | — |
 | `Alt+S` | Set root and scale | — |
 | `1`–`9` | Grid size — REAPER's own grid actions | same |
@@ -149,6 +150,22 @@ survive. They also need no mode: `L` behaves identically whether hold is on or
 off, so there is no state to set up beforehand and none to remember to turn
 off afterwards.
 
+## Hearing it in context
+
+`Alt+Space` plays from the **top of the bar the cursor is in**, and says which
+bar. Stop with `Space` as usual.
+
+Auditioning a cell tells you what a note *is*; playing from the downbeat tells
+you whether it is in the right *place*. At 16ths or 32nds that difference is
+the whole question, and it cannot be judged from a note in isolation.
+
+Your position is preserved. The edit cursor doubles as the grid cursor, so it
+is moved to the bar line only long enough to start playback and then put
+straight back — you carry on editing exactly where you were.
+
+This one works whether or not grid mode is on, since it is a listening aid
+rather than an editing action.
+
 ## Shifting notes in time
 
 If a whole clip landed a couple of cells off the beat, `Ctrl+Shift+Left` and