Themes: User themes cannot include "global" include files

Change https://codereview.qt-project.org/c/qt-creator/qt-creator/+/532065 taught .creatortheme how include files.

One use-case needs is not yet addressed: User themes, located in %APPDATA%\QtProject\qtcreator\themes or ~/.config/QtProject/ might want to include an include file from the Qt Creator installation themes directory.
For that case, there should be a mechanism that first tries to resolve the relative path withing the user themes directory and if the file does not exist, try to resolve it in the installation directory.