Qmake: Improve the build dir location warning

The infamous "build dir is not at the same level at source dir" warning
for qmake projects has gone through a number of iterations in Qt
Creator, having been added, removed and re-added, always to the
criticism of some users. Our new approach is as follows:
    - The warning appears at the widgets where the build directory is
set, both in the target setup page and the build config widget.
    - The warning also appears in the issues pane, but only if the build
failed.
    - The user can disable the warning altogether in a newly introduced
qmake settings page.
    - This option is disabled by default on Unix, because to my
knowledge all failure reports have been for Windows hosts.
This should finally please everybody.

Fixes: QTCREATORBUG-16945
Change-Id: I638be1f15e8c260a5d72047d6850a3a0f685cf03
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

src/plugins/qmakeprojectmanager/qmakeprojectmanagerplugin.cpp

@@ -28,6 +28,7 @@
 #include "profileeditor.h"
 #include "qmakeprojectmanager.h"
 #include "qmakenodes.h"
+#include "qmakesettings.h"
 #include "qmakestep.h"
 #include "qmakemakestep.h"
 #include "qmakebuildconfiguration.h"
@@ -102,6 +103,8 @@ public:
 
     ProFileEditorFactory profileEditorFactory;
 
+    QmakeSettingsPage settingsPage;
+
     ExternalQtEditor *m_designerEditor{ExternalQtEditor::createDesignerEditor()};
     ExternalQtEditor *m_linguistEditor{ExternalQtEditor::createLinguistEditor()};