Case sensitive files are colliding

Hi Bjarne,

when cloning your repo on windows, git says:


```
warning: the following paths have collided (e.g. case-sensitive paths
on a case-insensitive filesystem) and only one from the same
colliding group is in the working tree:

  'GUI.h'
  'Gui.h'
```


Can you fix this? Would be pretty useful ;)

Have a nice day!