Ensure qgis.db is writable when copied from RO source

In some package managers (such as NIX[1], used in NixOS[2]), all the data is
installed into a read-only file system.

When QGis is first launched on those systems, it copies the template
`qgis.db` from the share directory into the user private .local
directory, but keeps its permissions unchanged: readonly. Any attempt
from QGIS to update this file fails.

This commit makes sure that the `qgis.db` file is user writable when it
is copied from the installation directory.

[1] https://nixos.org/nix/
[2] https://nixos.org/

src/core/qgsapplication.cpp

@@ -1536,6 +1536,19 @@ bool QgsApplication::createDatabase( QString *errorMessage )
       }
       return false;
     }
+
+    QFile::Permissions perms = QFile( qgisPrivateDbFile.fileName() ).permissions();
+    if ( !( perms & QFile::WriteOwner ) )
+    {
+      if ( !qgisPrivateDbFile.setPermissions( perms | QFile::WriteOwner ) )
+      {
+        if ( errorMessage )
+        {
+          *errorMessage = tr( "Can not make '%1' user writable" ).arg( qgisPrivateDbFile.fileName() );
+        }
+        return false;
+      }
+    }
   }
   else
   {