Common prefixes are no longer removed from file groups

In Qt Creator <= 11.0.3 the Projects panel would automatically skip showing common prefix directories and only show the deepest view required to differentiate files (this is with Simplify View turned off).

The issue can be demonstrated with a CMake like so

add_executable(prefix_test
    src/core.h
    src/core.cpp
    src/referenceelectionconfig.h
    src/referenceelectionconfig.cpp
    src/resultspresenter.h
    src/resultspresenter.cpp
)

In this case "src" is the common prefix.

In Creator 11.0.3 this displays as:

where as in 12.0.0 is shows as:

As you can see the "redundant" 'src' directory is shown in 12.0.0. In this simple example this isn't a huge deal, but this extends to large examples like:

add_executable(prefix_test
    include/namespace/component/subcomponent1/header.h
    include/namespace/component/subcomponent2/header.h
)

where you need the lengthy structure for organization purposes (e.g. public API headers)

but many of the root folders are empty/all the same. In 11.0.3 creator would drop `include/namespace/component` and only show `subcomponent/header.h` and `subcomponent2/header.h` in the "Include Files" source_group I use (or just Headers if not using a source group), but in 12.0.0 you have to expand every subfolder shown to get to the actually files (regardless of if source_group is used).

I'm not sure if this was intentional or not (I'll try to bisect this later), but I prefer the old behavior if it was.