Allow to specify default argument for layout/inset argments

This default argument is inserted iff no inset argument is present. This is useful particularly for mandatory arguments that need to have a sensible default value.

lib/doc/Customization.lyx

@@ -1,5 +1,5 @@
 #LyX 2.1 created this file. For more info see http://www.lyx.org/
-\lyxformat 461
+\lyxformat 463
 \begin_document
 \begin_header
 \textclass scrbook
@@ -11024,7 +11024,44 @@ status collapsed
 
 \begin_layout Itemize
 
-\change_inserted -712698321 1355144578
+\change_inserted -712698321 1361701444
+\begin_inset Flex Code
+status collapsed
+
+\begin_layout Plain Layout
+
+\change_inserted -712698321 1361701356
+DefaultArg
+\end_layout
+
+\end_inset
+
+ 
+\begin_inset Flex Code
+status collapsed
+
+\begin_layout Plain Layout
+
+\change_inserted -712698321 1361701356
+[string]
+\end_layout
+
+\end_inset
+
+ defines an argument that is inserted if and only if no user-specified arguments
+ were given, i.
+\begin_inset space \thinspace{}
+\end_inset
+
+e.
+ if no argument inset has been inserted (note that also an empty argument
+ inset omits the DefaultArg).
+ Multiple arguments need to be separated by comma.
+\end_layout
+
+\begin_layout Itemize
+
+\change_inserted -712698321 1361701337
 \begin_inset Flex Code
 status collapsed
 
@@ -11053,8 +11090,6 @@ status collapsed
  defines an argument that is inserted in any case (alone or in addition
  to user-specified arguments).
  Multiple arguments need to be separated by comma.
-\change_unchanged
-
 \end_layout
 
 \begin_layout Itemize

src/Layout.cpp

@@ -923,6 +923,9 @@ void Layout::readArgument(Lexer & lex)
 			arg.rdelim = lex.getDocString();
 			arg.rdelim = support::subst(arg.rdelim, from_ascii("<br/>"),
 						    from_ascii("\n"));
+		} else if (tok == "defaultarg") {
+			lex.next();
+			arg.defaultarg = lex.getDocString();
 		} else if (tok == "presetarg") {
 			lex.next();
 			arg.presetarg = lex.getDocString();

src/Layout.h

@@ -96,6 +96,7 @@ public:
 		bool mandatory;
 		docstring ldelim;
 		docstring rdelim;
+		docstring defaultarg;
 		docstring presetarg;
 		docstring tooltip;
 		std::string requires;

src/insets/InsetLayout.cpp

@@ -551,6 +551,9 @@ void InsetLayout::readArgument(Lexer & lex)
 			arg.rdelim = lex.getDocString();
 			arg.rdelim = support::subst(arg.rdelim,
 						    from_ascii("<br/>"), from_ascii("\n"));
+		} else if (tok == "defaultarg") {
+			lex.next();
+			arg.defaultarg = lex.getDocString();
 		} else if (tok == "presetarg") {
 			lex.next();
 			arg.presetarg = lex.getDocString();

src/output_latex.cpp

@@ -351,22 +351,24 @@ void getArgInsets(otexstream & os, OutputParams const & runparams, Layout::LaTeX
 				string const name = prefix + convert<string>(i);
 				if ((*lait).first == name) {
 					Layout::latexarg arg = (*lait).second;
+					docstring preset = arg.presetarg;
+					if (!arg.defaultarg.empty()) {
+						if (!preset.empty())
+							preset += ",";
+						preset += arg.defaultarg;
+					}
 					if (arg.mandatory) {
 						docstring ldelim = arg.ldelim.empty() ?
 								from_ascii("{") : arg.ldelim;
 						docstring rdelim = arg.rdelim.empty() ?
 								from_ascii("}") : arg.rdelim;
-						os << ldelim << arg.presetarg << rdelim;
-					} else if (!arg.presetarg.empty()) {
-						docstring ldelim = arg.mandatory ?
-								from_ascii("{") : from_ascii("[");
-						docstring rdelim = arg.mandatory ?
-								from_ascii("}") : from_ascii("]");
-						if (!arg.ldelim.empty())
-							ldelim = arg.ldelim;
-						if (!arg.rdelim.empty())
-							rdelim = arg.rdelim;
-						os << ldelim << arg.presetarg << rdelim;
+						os << ldelim << preset << rdelim;
+					} else if (!preset.empty()) {
+						docstring ldelim = arg.ldelim.empty() ?
+								from_ascii("[") : arg.ldelim;
+						docstring rdelim = arg.rdelim.empty() ?
+								from_ascii("]") : arg.rdelim;
+						os << ldelim << preset << rdelim;
 					} else if (find(required.begin(), required.end(),
 						   (*lait).first) != required.end()) {
 						docstring ldelim = arg.ldelim.empty() ?