Add delete to the selection bar and drop the in-header close button

The selection bar covered copying but not the other action people reach
for with a selection. The header also carried a second close button next
to the window's own traffic light, so it was removed in favor of that.

README.md

@@ -5,8 +5,8 @@ A native macOS app for keeping prompts and photos, in the spirit of
 to your work. Select what you need and send it back to your clipboard.
 
 - **Click to select** — clicking a card selects it (Shift+click for a range),
-  and the selection bar copies a single item as-is, or the whole selection as a
-  numbered list or as plain newline-separated lines.
+  and the selection bar copies a single item as-is, the whole selection as a
+  numbered list or as plain newline-separated lines, or deletes it.
 - **Text or photo** — copying sends text back as text and a photo back as an
   image.
 - **Capture from anywhere** — select text in any app and tap **Left Shift twice**.