support undo of paste button

0067-paste_button

@@ -1,7 +1,20 @@
 paste_button = function(x, y, r)
 	return overflowable_button('paste', x, y, r,
 		function()
+			local state = Current_pane.editor_state
+			-- initial state for recording undo operation
+			local before_line = state.cursor1.line
+			local before = snapshot(state, before_line)
+			-- paste
 			local s = App.get_clipboard()
-			Text.insert_text(Current_pane.editor_state, s)
+			Text.insert_text(state, s)
+			-- record undo operation
+			record_undo_event(state,
+				{
+					before=before,
+					after=snapshot(
+						state, before_line,
+						state.cursor1.line)
+				})
 		end)
-end
+end