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.cpp

@@ -88,6 +88,15 @@ QVariant ConfigModel::data(const QModelIndex &index, int role) const
 
     const InternalDataItem &item = m_configuration[index.row()];
 
+    if (index.column() < 2) {
+        switch (role) {
+        case ItemTypeRole:
+            return item.type;
+        case ItemValuesRole:
+            return item.values;
+        }
+    }
+
     switch (index.column()) {
     case 0:
         switch (role) {
@@ -97,8 +106,6 @@ QVariant ConfigModel::data(const QModelIndex &index, int role) const
             return item.key;
         case Qt::ToolTipRole:
             return item.description;
-        case Qt::UserRole:
-            return item.type;
         case Qt::FontRole: {
             QFont font;
             font.setItalic(item.isCMakeChanged);
@@ -126,8 +133,6 @@ QVariant ConfigModel::data(const QModelIndex &index, int role) const
         }
         case Qt::ToolTipRole:
             return item.description;
-        case Qt::UserRole:
-            return item.type;
         default:
             return QVariant();
         }
@@ -209,13 +214,15 @@ QVariant ConfigModel::headerData(int section, Qt::Orientation orientation, int r
 void ConfigModel::appendConfiguration(const QString &key,
                                       const QString &value,
                                       const ConfigModel::DataItem::Type type,
-                                      const QString &description)
+                                      const QString &description,
+                                      const QStringList &values)
 {
     DataItem item;
     item.key = key;
     item.type = type;
     item.value = value;
     item.description = description;
+    item.values = values;
 
     InternalDataItem internalItem(item);
     internalItem.isUserNew = true;