Only expect a single 'did-destroy' event

These events will only be fired a single time at most, so we should clean up the listeners after that.

This should help minimize accidental memory leaks.

src/cursor.coffee

@@ -49,7 +49,7 @@ class Cursor extends Model
   #
   # Returns a {Disposable} on which `.dispose()` can be called to unsubscribe.
   onDidDestroy: (callback) ->
-    @emitter.on 'did-destroy', callback
+    @emitter.once 'did-destroy', callback
 
   ###
   Section: Managing Cursor Position

src/decoration.coffee

@@ -105,7 +105,7 @@ class Decoration
   #
   # Returns a {Disposable} on which `.dispose()` can be called to unsubscribe.
   onDidDestroy: (callback) ->
-    @emitter.on 'did-destroy', callback
+    @emitter.once 'did-destroy', callback
 
   ###
   Section: Decoration Details

src/git-repository.coffee

@@ -129,7 +129,7 @@ class GitRepository
   #
   # Returns a {Disposable} on which `.dispose()` can be called to unsubscribe.
   onDidDestroy: (callback) ->
-    @emitter.on 'did-destroy', callback
+    @emitter.once 'did-destroy', callback
 
   ###
   Section: Event Subscription

src/gutter.coffee

@@ -48,7 +48,7 @@ class Gutter
   #
   # Returns a {Disposable} on which `.dispose()` can be called to unsubscribe.
   onDidDestroy: (callback) ->
-    @emitter.on 'did-destroy', callback
+    @emitter.once 'did-destroy', callback
 
   ###
   Section: Visibility

src/pane-axis.coffee

@@ -69,7 +69,7 @@ class PaneAxis extends Model
     @emitter.on 'did-replace-child', fn
 
   onDidDestroy: (fn) ->
-    @emitter.on 'did-destroy', fn
+    @emitter.once 'did-destroy', fn
 
   onDidChangeFlexScale: (fn) ->
     @emitter.on 'did-change-flex-scale', fn

src/pane.coffee

@@ -170,7 +170,7 @@ class Pane
   #
   # Returns a {Disposable} on which `.dispose()` can be called to unsubscribe.
   onDidDestroy: (callback) ->
-    @emitter.on 'did-destroy', callback
+    @emitter.once 'did-destroy', callback
 
   # Public: Invoke the given callback when the value of the {::isActive}
   # property changes.

src/panel-container.js

@@ -40,7 +40,7 @@ module.exports = class PanelContainer {
   }
 
   onDidDestroy (callback) {
-    return this.emitter.on('did-destroy', callback)
+    return this.emitter.once('did-destroy', callback)
   }
 
   /*

src/panel.js

@@ -66,7 +66,7 @@ class Panel {
   //
   // Returns a {Disposable} on which `.dispose()` can be called to unsubscribe.
   onDidDestroy (callback) {
-    return this.emitter.on('did-destroy', callback)
+    return this.emitter.once('did-destroy', callback)
   }
 
   /*

src/selection.coffee

@@ -54,7 +54,7 @@ class Selection extends Model
   #
   # Returns a {Disposable} on which `.dispose()` can be called to unsubscribe.
   onDidDestroy: (callback) ->
-    @emitter.on 'did-destroy', callback
+    @emitter.once 'did-destroy', callback
 
   ###
   Section: Managing the selection range

src/text-editor.coffee

@@ -647,7 +647,7 @@ class TextEditor extends Model
   #
   # Returns a {Disposable} on which `.dispose()` can be called to unsubscribe.
   onDidDestroy: (callback) ->
-    @emitter.on 'did-destroy', callback
+    @emitter.once 'did-destroy', callback
 
   # Extended: Calls your `callback` when a {Cursor} is added to the editor.
   # Immediately calls your callback for each existing cursor.