Add dialog API.

Supported APIs:
* openFile
* openMultiFiles
* openFolder
* saveAs

Each API is asynchronous and accepts a callback.

renderer/api/lib/dialog.coffee

@@ -0,0 +1,41 @@
+ipc = require 'ipc'
+
+callbackId = 0
+callbacks = {}
+
+storeCallback = (callback) ->
+  throw new TypeError('Bad argument') unless typeof callback is 'function'
+
+  ++callbackId
+  callbacks[callbackId] = callback
+  callbackId
+
+makeCallback = (id, args...) ->
+  callbacks[id].call global, args...
+  delete callbacks[id]
+
+# Force loading dialog code in browser.
+remote.require 'dialog'
+
+ipc.on 'ATOM_RENDERER_DIALOG', (id, args...) ->
+  makeCallback(id, args...)
+
+callFileDialogs = (options, callback, args...) ->
+  if typeof options is 'function'
+    callback = options
+    options = {}
+
+  ipc.sendChannel 'ATOM_BROWSER_FILE_DIALOG', storeCallback(callback), args..., options
+
+module.exports =
+  openFolder: (options, callback) ->
+    callFileDialogs options, callback, 1, 'Open Folder'
+
+  saveAs: (options, callback) ->
+    callFileDialogs options, callback, 2, 'Save As'
+
+  openFile: (options, callback) ->
+    callFileDialogs options, callback, 3, 'Open File'
+
+  openMultiFiles: (options, callback) ->
+    callFileDialogs options, callback, 4, 'Open Files'