refactor: add move constructor and move assignment operator to ValueD…

…esc (synfig#3150)

Note: sadly the usage is low.
By adding debug messages on copy constructor and assignment operator,
it can be seen there is *a lot* of copy while clicking around, moving
mouse or even after a Alt+Tab to change to other application.

synfig-studio/src/synfigapp/value_desc.h

@@ -136,6 +136,34 @@ class ValueDesc
 		return *this;
 	}
 
+	ValueDesc& operator=(ValueDesc&& other) noexcept
+	{
+		if (this == &other)
+			return *this;
+
+		if (parent_desc && 0 >= --parent_desc->links_count)
+			delete parent_desc;
+
+		if (id_changed_connection.connected())
+			id_changed_connection.disconnect();
+
+		layer = std::move(other.layer);
+		name = std::move(other.name);
+		parent_value_node = std::move(other.parent_value_node);
+		index = other.index;
+		waypoint_time = std::move(other.waypoint_time);
+		canvas = std::move(other.canvas);
+		id_changed_connection = std::move(other.id_changed_connection);
+		sub_names = std::move(other.sub_names);
+		parent_desc = std::move(other.parent_desc);
+		links_count = other.links_count;
+
+		other.parent_desc = nullptr;
+		other.links_count = 0;
+
+		return *this;
+	}
+
 	ValueDesc(synfig::Layer::Handle layer,const synfig::String& param_name,const ValueDesc &parent = blank):
 		layer(layer),
 		name(param_name),
@@ -217,6 +245,15 @@ class ValueDesc
 		if (parent_desc) parent_desc->links_count++;
 	}
 
+	// move
+	ValueDesc(ValueDesc&& other) noexcept
+		: index(IS_CONST),
+		  parent_desc(nullptr),
+		  links_count(0)
+	{
+		*this = std::move(other);
+	}
+
 	ValueDesc():
 		index(IS_CONST), parent_desc(nullptr), links_count(0) { }