Selections: add captures field

Selections now can have associated captures, for backreferences when
selections are created from a regex search

src/window.cc

@@ -28,6 +28,13 @@ void Selection::merge_with(const Selection& selection)
     m_last = selection.m_last;
 }
 
+BufferString Selection::capture(size_t index) const
+{
+    if (index < m_captures.size())
+        return m_captures[index];
+    return "";
+}
+
 struct scoped_undo_group
 {
     scoped_undo_group(Buffer& buffer)

src/window.hh

@@ -14,8 +14,15 @@ namespace Kakoune
 
 struct Selection
 {
-    Selection(const BufferIterator& first, const BufferIterator& last)
-        : m_first(first), m_last(last) {}
+    typedef std::vector<BufferString> CaptureList;
+
+    Selection(const BufferIterator& first, const BufferIterator& last,
+              const CaptureList& captures = CaptureList())
+        : m_first(first), m_last(last), m_captures(captures) {}
+
+    Selection(const BufferIterator& first, const BufferIterator& last,
+              CaptureList&& captures)
+        : m_first(first), m_last(last), m_captures(captures) {}
 
     BufferIterator begin() const;
     BufferIterator end() const;
@@ -25,9 +32,14 @@ struct Selection
 
     void merge_with(const Selection& selection);
 
+    BufferString capture(size_t index) const;
+    const CaptureList& captures() const { return m_captures; }
+
 private:
     DynamicBufferIterator m_first;
     DynamicBufferIterator m_last;
+
+    CaptureList m_captures;
 };
 
 typedef std::vector<Selection> SelectionList;