QtSupport: allow relative paths for the qmake command

This change makes it possible to bake in settings for bundled/shipped
Qt versions like we have in the QtDesignStudio.app package.
The result is that the installer does not need to write absolute
paths to the settings anymore, so it stays like it was signed.

A signed package on macOS is more and more necessary
and recommended for a while. Especially If the application will
touch files in special folders like Download, Documents, or Desktop.

Change-Id: I3153ffd4229b34a59bdfe740937c21ee93de3e9b
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/libs/utils/persistentsettings.cpp

@@ -336,11 +336,17 @@ bool PersistentSettingsReader::load(const FilePath &fileName)
     if (fileName.fileSize() == 0) // skip empty files
         return false;
 
+    m_filePath = fileName.parentDir();
     ParseContext ctx;
     m_valueMap = ctx.parse(fileName);
     return true;
 }
 
+FilePath PersistentSettingsReader::filePath()
+{
+    return m_filePath;
+}
+
 /*!
     \class Utils::PersistentSettingsWriter