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/cmakeconfigitem.cpp

@@ -42,7 +42,7 @@ CMakeConfigItem::CMakeConfigItem() = default;
 
 CMakeConfigItem::CMakeConfigItem(const CMakeConfigItem &other) :
     key(other.key), type(other.type), isAdvanced(other.isAdvanced),
-    value(other.value), documentation(other.documentation)
+    value(other.value), documentation(other.documentation), values(other.values)
 { }
 
 CMakeConfigItem::CMakeConfigItem(const QByteArray &k, Type t,