Use Core::Id in project-related objects

Use Core::Id for all the project related objects in favor of plain
QStrings.

Change-Id: I790ab40cb29899efdb49c413a77609486f52e683
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>

src/plugins/projectexplorer/toolchain.cpp

@@ -156,9 +156,9 @@ QString ToolChain::id() const
     An empty list is shows that the toolchain is compatible with all targets.
 */
 
-QStringList ToolChain::restrictedToTargets() const
+QList<Core::Id> ToolChain::restrictedToTargets() const
 {
-    return QStringList();
+    return QList<Core::Id>();
 }
 
 QList<Utils::FileName> ToolChain::mkspecList() const