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 <[email protected]>

headers/build/private/app/AppMisc.h

@@ -1,28 +1,11 @@
-//------------------------------------------------------------------------------
-//	Copyright (c) 2001-2002, OpenBeOS
-//
-//	Permission is hereby granted, free of charge, to any person obtaining a
-//	copy of this software and associated documentation files (the "Software"),
-//	to deal in the Software without restriction, including without limitation
-//	the rights to use, copy, modify, merge, publish, distribute, sublicense,
-//	and/or sell copies of the Software, and to permit persons to whom the
-//	Software is furnished to do so, subject to the following conditions:
-//
-//	The above copyright notice and this permission notice shall be included in
-//	all copies or substantial portions of the Software.
-//
-//	THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-//	IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-//	FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-//	AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-//	LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
-//	FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
-//	DEALINGS IN THE SOFTWARE.
-//
-//	File Name:		AppMisc.h
-//	Author:			Ingo Weinhold ([email protected])
-//	Description:	Miscellaneous private functionality.
-//------------------------------------------------------------------------------
+/*
+ * Copyright 2001-2019 Haiku, Inc. All rights reserved.
+ * Distributed under the terms of the MIT License.
+ *
+ * Authors:
+ *		Ingo Weinhold, [email protected]
+ *		Jacob Secunda
+ */
 
 #ifndef _APP_MISC_H
 #define _APP_MISC_H
@@ -45,8 +28,6 @@ thread_id main_thread_for(team_id team);
 
 bool is_running_on_haiku();
 
-bool is_app_showing_modal_window(team_id team);
-
 } // namespace BPrivate
 
 // _get_object_token_