Add an option to show files without dividing them to Headers/Sources/Forms

This additional level of hierarchy (Source/Headers) is inconvenient. I expect similarly named files (whatever.h, whatever.cpp, whatever.inl) to be close to each other in solution navigator sidebar, but right now it requires me to keep the same directory structure opened twice.

Just add an option to show files as they are physically on the disk, without any additional segregation.