CMakeProjectManager: Support drop down selector for options

CMake provides "hack" for cmake-gui, that allows set options variants
and select then from drop down list. Allows Qt Creator re-use this
solution.

See:
- https://blog.kitware.com/constraining-values-with-comboboxes-in-cmake-cmake-gui/
- http://blog.bethcodes.com/cmake-tips-tricks-drop-down-list

Drop down values can be added to option via:
 SET_PROPERTY(CACHE OptionName PROPERTY STRINGS Option1 Option2 Option3)

This solution should not restrict to provide any other value, it
provides only suggestion for user to select one of prdefined values.

Change-Id: I8fc52155775f1e04979db8206bb42363df9359e8
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/cmakeprojectmanager/configmodel.h

@@ -34,6 +34,11 @@ class ConfigModel : public QAbstractTableModel
     Q_OBJECT
 
 public:
+    enum Roles {
+        ItemTypeRole = Qt::UserRole,
+        ItemValuesRole
+    };
+
     class DataItem {
     public:
         enum Type { BOOLEAN, FILE, DIRECTORY, STRING, UNKNOWN};
@@ -43,6 +48,7 @@ public:
         bool isAdvanced = false;
         QString value;
         QString description;
+        QStringList values;
     };
 
     explicit ConfigModel(QObject *parent = nullptr);
@@ -58,7 +64,8 @@ public:
     void appendConfiguration(const QString &key,
                              const QString &value = QString(),
                              const DataItem::Type type = DataItem::UNKNOWN,
-                             const QString &description = QString());
+                             const QString &description = QString(),
+                             const QStringList &values = QStringList());
     void setConfiguration(const QList<DataItem> &config);
     void flush();
     void resetAllChanges();