[LyX/master] Remove heavy includes

[Georg Baum]

commit 38d387d1ab6c1ed11d845996de768433feb18f20
Author: Georg Baum <b...@lyx.org>
Date:   Sun Nov 30 12:52:01 2014 +0100

    Remove heavy includes

diff --git a/src/frontends/alert.h b/src/frontends/alert.h
index b96ca1e..2be612a 100644
--- a/src/frontends/alert.h
+++ b/src/frontends/alert.h
@@ -12,7 +12,7 @@
 #ifndef LYX_ALERT_H
 #define LYX_ALERT_H
 
-#include "support/docstring.h"
+#include "support/strfwd.h"
 
 
 namespace lyx {
@@ -33,8 +33,8 @@ namespace Alert {
 int prompt(docstring const & title, docstring const & question,
           int default_button, int cancel_button,
           docstring const & b1, docstring const & b2,
-          docstring const & b3 = docstring(),
-          docstring const & b4 = docstring());
+          docstring const & b3 = empty_docstring(),
+          docstring const & b4 = empty_docstring());
 
 /**
  * Display a warning to the user. Title should be a short (general) summary.
@@ -65,7 +65,7 @@ void information(docstring const & title, docstring const & 
message);
  * (even empty string). dflt stands for default message in the dialog.
  */
 bool askForText(docstring & response, docstring const & msg,
-       docstring const & dflt = docstring());
+       docstring const & dflt = empty_docstring());
 
 } // namespace Alert
 } // namespace frontend
diff --git a/src/mathed/InsetMathUnknown.h b/src/mathed/InsetMathUnknown.h
index 0436be5..2effe3c 100644
--- a/src/mathed/InsetMathUnknown.h
+++ b/src/mathed/InsetMathUnknown.h
@@ -14,8 +14,6 @@
 
 #include "InsetMath.h"
 
-#include "support/docstring.h"
-
 
 namespace lyx {
 
@@ -24,7 +22,7 @@ class InsetMathUnknown : public InsetMath {
 public:
        ///
        explicit InsetMathUnknown(docstring const & name,
-               docstring const & selection = docstring(),
+               docstring const & selection = empty_docstring(),
                bool final = true, bool black = false);
        ///
        void metrics(MetricsInfo & mi, Dimension & dim) const;