Add-ons: const id; adjust the Addon constructors, remove Addon::setId()

Since the new _storagePath data member internally contains the add-on
id, changing _id after _storagePath has been initialized would make both
data members inconsistent. As changing the add-on id is probably not a
very useful operation, the simplest way to prevent such an inconsistency
from happening is to make Addon's _id data member const (as is already
the case for _storagePath), and thus remove Addon::setId().
Consequently, remove the Addon default constructor too, since add-ons
with an empty id would be ill-formed and couldn't be changed (_id being
const now). This leaves us with one Addon constructor:

Addon(std::string id, AddonVersion version = AddonVersion(),
      SGPath basePath = SGPath(), std::string minFGVersionRequired = "",
      std::string maxFGVersionRequired = "",
      SGPropertyNode* addonNode = nullptr);

src/Add-ons/Addon.cxx

@@ -110,17 +110,9 @@ Addon::Addon(std::string id, AddonVersion version, SGPath basePath,
   }
 }
 
-Addon::Addon()
-  : Addon(std::string(), AddonVersion(), SGPath(), std::string(),
-          std::string(), nullptr)
-{ }
-
 std::string Addon::getId() const
 { return _id; }
 
-void Addon::setId(const std::string& addonId)
-{ _id = addonId; }
-
 std::string Addon::getName() const
 { return _name; }
 

src/Add-ons/Addon.hxx

@@ -77,13 +77,10 @@ private:
 class Addon : public SGReferenced
 {
 public:
-  // Default constructor. 'minFGVersionRequired' is initialized to "2017.4.0"
-  // and 'maxFGVersionRequired' to "none".
-  Addon();
   // An empty value for 'minFGVersionRequired' is translated into "2017.4.0".
   // An empty value for 'maxFGVersionRequired' is translated into "none".
-  Addon(std::string id, AddonVersion version, SGPath basePath,
-        std::string minFGVersionRequired = "",
+  Addon(std::string id, AddonVersion version = AddonVersion(),
+        SGPath basePath = SGPath(), std::string minFGVersionRequired = "",
         std::string maxFGVersionRequired = "",
         SGPropertyNode* addonNode = nullptr);
 
@@ -92,7 +89,6 @@ public:
   static Addon fromAddonDir(const SGPath& addonPath);
 
   std::string getId() const;
-  void setId(const std::string& addonId);
 
   std::string getName() const;
   void setName(const std::string& addonName);
@@ -205,7 +201,7 @@ private:
 
   // The add-on identifier, in reverse DNS style. The AddonManager refuses to
   // register two add-ons with the same id in a given FlightGear session.
-  std::string _id;
+  const std::string _id;
   // Pretty name for the add-on (not constrained to reverse DNS style)
   std::string _name;
   // Use a smart pointer to expose the AddonVersion instance to Nasal without

src/Add-ons/test_AddonManagement.cxx

@@ -209,9 +209,8 @@ void testAddon()
 {
   fgtest::initTestGlobals("Addon");
 
-  Addon addon;
   std::string addonId = "org.FlightGear.addons.MyGreatAddon";
-  addon.setId(addonId);
+  Addon addon{addonId};
   addon.setVersion(AddonVersion("2017.2.5rc3"));
   addon.setBasePath(SGPath("/path/to/MyGreatAddon"));
   addon.setMinFGVersionRequired("2017.4.1");