CMake: Move headers into their own folder in server-mode

Move all project headers into one folder. This reduces the time it
takes to generate the project tree, since the headers will only show up
once now (instead of once per target), severly reducing the number of
nodes in the tree.

Change-Id: Ibcfa7c02c1aec4a98054f4f8a97b69dfb4c25ae4
Reviewed-by: hjk <hjk@qt.io>