Pointer cleanup (#242)

- Made greater use of smart pointers, eliminating many instances of manual memory management
- Do not use pointers at all for many non-polymorphic classes
- Assorted other code cleanup

model/Distance.cpp

@@ -56,7 +56,7 @@ namespace glabels
 		}
 
 
-		Distance::Distance( double d, const Units& units )
+		Distance::Distance( double d, Units units )
 		{
 			switch (units.toEnum())
 			{
@@ -128,7 +128,7 @@ namespace glabels
 		}
 
 
-		double Distance::inUnits( const Units& units ) const
+		double Distance::inUnits( Units units ) const
 		{
 			double d;
 
@@ -188,7 +188,7 @@ namespace glabels
 		}
 
 
-		QString Distance::toString( const Units& units ) const
+		QString Distance::toString( Units units ) const
 		{
 			return QString::number( inUnits(units) ) + units.toIdString();
 		}
@@ -211,7 +211,7 @@ namespace glabels
 }
 
 
-QDebug operator<<( QDebug dbg, const glabels::model::Distance& distance )
+QDebug operator<<( QDebug dbg, glabels::model::Distance distance )
 {
 	QDebugStateSaver saver(dbg);