Added support for STRINGS property in CMake cache editor

Summary:
CMake had support of options with a dropdown choice since version 2.8. This patch add support for displaying cmake variable that has a STRINGS property as an editable
dropdown that mimic the behavior of cmake-gui.

Here's an example of a cmake dropdown option:

set(Color "Red" CACHE STRING
  "Color chosen by the user at CMake configure time")

set(ColorValues "Red;Orange;Yellow;Green;Blue;Violet" CACHE STRING
  "List of possible values for the Color cache variable")

set_property(CACHE Color PROPERTY STRINGS ${ColorValues})

All other info about cmake STRINGS property on this kitware blog post and on the cmake documentation

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D8215