Add some comments about the behavior of cellInset and cellIndex.
CMake: add option to enable address sanitizer
Cmake build: Move use of QT5 libraries to the end of linker command line.

To prevent the linker error if they were before some object file referencing
some symbol there.
Use a const ref here. Guess it was a thinko when I did it.
Make the constructor of the singleton class LayoutFileList private.
Little bit of cleanup.
Update comments.