Implements multiple selections clipboard paste

Includes:

  - Passing the selection index in the `Editor::mutateSelectedText`
method callback
  - Storing all the selections content on many calls of
`Selection::copy` with `maintainClipboard = true` in a metadata
`selections` array
  - Handling clipboard with a `selections` metadata in
the`Editor::pasteText` method

src/selection.coffee

@@ -503,11 +503,24 @@ class Selection extends Model
     @delete()
 
   # Public: Copies the current selection to the clipboard.
+  #
+  # If the `maintainClipboard` is set to `true`, a specific metadata property
+  # is created to store each content copied to the clipboard. The clipboard
+  # `text` still contains the concatenation of the clipboard with the
+  # current selection.
   copy: (maintainClipboard=false) ->
     return if @isEmpty()
     text = @editor.buffer.getTextInRange(@getBufferRange())
     if maintainClipboard
-      text = "#{atom.clipboard.read()}\n#{text}"
+      {text: clipboardText, metadata: clipboardMetadata} = atom.clipboard.readWithMetadata()
+
+      if clipboardMetadata? and clipboardMetadata.selections?
+        metadata = clipboardMetadata
+        clipboardMetadata.selections.push(text)
+      else
+        metadata = { selections: [clipboardText, text] }
+
+      text = "" + (clipboardText) + "\n" + text
     else
       metadata = { indentBasis: @editor.indentationForBufferRow(@getBufferRange().start.row) }