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/editor.coffee

@@ -725,13 +725,24 @@ class Editor extends Model
   # Public: For each selection, replace the selected text with the contents of
   # the clipboard.
   #
+  # If the clipboard contains the same number of selection as the current
+  # editor, each selection will be replaced with the content of the
+  # corresponding clipboard selection text.
+  #
   # options - See {Selection::insertText}.
   pasteText: (options={}) ->
     {text, metadata} = atom.clipboard.readWithMetadata()
 
     containsNewlines = text.indexOf('\n') isnt -1
 
-    if atom.config.get('editor.normalizeIndentOnPaste') and metadata
+    if metadata?.selections? and metadata.selections.length is @getSelections().length
+      @mutateSelectedText (selection, index) =>
+        text = metadata.selections[index]
+        selection.insertText(text, options)
+
+      return
+
+    else if atom.config.get("editor.normalizeIndentOnPaste") and metadata?.indentBasis?
       if !@getCursor().hasPrecedingCharactersOnLine() or containsNewlines
         options.indentBasis ?= metadata.indentBasis
 
@@ -1007,7 +1018,7 @@ class Editor extends Model
   #
   # fn - A {Function} that will be called with each {Selection}.
   mutateSelectedText: (fn) ->
-    @transact => fn(selection) for selection in @getSelections()
+    @transact => fn(selection,index) for selection,index in @getSelections()
 
   replaceSelectedText: (options={}, fn) ->
     {selectWordIfEmpty} = options

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) }