New class QgsMargins for storing margins (left/right/top/bottom)

Basically a direct port of QMarginF, but forced to always use
double values, and with added toString()/fromString() methods.

python/core/core.sip

@@ -93,6 +93,7 @@
 %Include qgsmapsettings.sip
 %Include qgsmaptopixel.sip
 %Include qgsmapunitscale.sip
+%Include qgsmargins.sip
 %Include qgsmessagelog.sip
 %Include qgsmessageoutput.sip
 %Include qgsmimedatautils.sip

python/core/qgsmargins.sip

@@ -0,0 +1,47 @@
+class QgsMargins
+{
+%TypeHeaderCode
+#include <qgsmargins.h>
+%End
+
+  public:
+
+    QgsMargins();
+    QgsMargins( double left, double top, double right, double bottom );
+
+    bool isNull() const;
+
+    double left() const;
+    double top() const;
+    double right() const;
+    double bottom() const;
+
+    void setLeft( double left );
+    void setTop( double top );
+    void setRight( double right );
+    void setBottom( double bottom );
+
+    QgsMargins &operator+=( const QgsMargins &margins );
+    QgsMargins &operator-=( const QgsMargins &margins );
+    QgsMargins &operator+=( double addend );
+    QgsMargins &operator-=( double subtrahend );
+    QgsMargins &operator*=( double factor );
+    QgsMargins &operator/=( double divisor );
+
+    QString toString() const;
+    static QgsMargins fromString( const QString& string );
+};
+
+bool operator==( const QgsMargins &lhs, const QgsMargins &rhs );
+bool operator!=( const QgsMargins &lhs, const QgsMargins &rhs );
+QgsMargins operator+( const QgsMargins &m1, const QgsMargins &m2 );
+QgsMargins operator-( const QgsMargins &m1, const QgsMargins &m2 );
+QgsMargins operator+( const QgsMargins &lhs, double rhs );
+QgsMargins operator+( double lhs, const QgsMargins &rhs );
+QgsMargins operator-( const QgsMargins &lhs, double rhs );
+QgsMargins operator*( const QgsMargins &margins, double factor );
+QgsMargins operator*( double factor, const QgsMargins &margins );
+QgsMargins operator/( const QgsMargins &margins, double divisor );
+QgsMargins operator+( const QgsMargins &margins );
+QgsMargins operator-( const QgsMargins &margins );
+

src/core/CMakeLists.txt

@@ -170,6 +170,7 @@ SET(QGIS_CORE_SRCS
   qgsmaptopixel.cpp
   qgsmaptopixelgeometrysimplifier.cpp
   qgsmapunitscale.cpp
+  qgsmargins.cpp
   qgsmessagelog.cpp
   qgsmessageoutput.cpp
   qgsmimedatautils.cpp
@@ -714,6 +715,7 @@ SET(QGIS_CORE_HDRS
   qgsmaptopixel.h
   qgsmaptopixelgeometrysimplifier.h
   qgsmapunitscale.h
+  qgsmargins.h
   qgsmimedatautils.h
   qgsmultirenderchecker.h
   qgsobjectcustomproperties.h

src/core/qgsmargins.cpp

@@ -0,0 +1,37 @@
+/***************************************************************************
+  qgsmargins.cpp
+  --------------
+  Date                 : January 2017
+  Copyright            : (C) 2017 by Nyall Dawson
+  Email                : nyall dot dawson at gmail dot com
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+
+#include "qgsmargins.h"
+
+QString QgsMargins::toString() const
+{
+  if ( isNull() )
+    return QString();
+  else
+    return QStringLiteral( "%1,%2,%3,%4" ).arg( qgsDoubleToString( mLeft ), qgsDoubleToString( mTop ),
+           qgsDoubleToString( mRight ), qgsDoubleToString( mBottom ) );
+}
+
+QgsMargins QgsMargins::fromString( const QString& string )
+{
+  QStringList margins = string.split( ',' );
+  if ( margins.count() != 4 )
+    return QgsMargins();
+
+  return QgsMargins( margins.at( 0 ).toDouble(),
+                     margins.at( 1 ).toDouble(),
+                     margins.at( 2 ).toDouble(),
+                     margins.at( 3 ).toDouble() );
+}