Implement is_app_showing_modal_window()

Also do some cleanup in private headers, I can't imagine why the build
libraries would need this function.

Change-Id: Ib08810b6efe4738dad596a735d741582a3781b

Implement is_app_showing_modal_window()

Also do some cleanup in private headers, I can't imagine why the build
libraries would need this function.

Change-Id: Ib08810b6efe4738dad596a735d741582a3781b28
Reviewed-on: https://review.haiku-os.org/c/1670
Reviewed-by: Ryan Leavengood <leavengood@gmail.com>

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Merged changes from branch build_system_redesign at revision 14573.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@14574 a95241bf-73f2-0310-859d-f6bbb57e9c96