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](https://blog.kitware.com/constraining-values-with-comboboxes-in-cmake-cmake-gui/) and on the [cmake documentation](https://cmake.org/cmake/help/latest/prop_cache/STRINGS.html)