Utils: Introduce a FilePath constructor from char arrays

Similar to QT_RESTRICTED_CAST_FROM_ASCII to avoid the need for
decorations in user code.

At the same time, drop some convenience constructors and functions
in CommandLine and Icon essentially serving the same purpose.

Change-Id: Ida4e5ac19c2da0a4298a97b2a8e1511d56bbb79d
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/studiowelcome/examplecheckout.cpp

@@ -219,7 +219,7 @@ FileExtractor::FileExtractor(QObject *parent)
     if (Core::DocumentManager::instance())
         m_targetPath = Core::DocumentManager::projectsDirectory();
     else
-        m_targetPath = Utils::FilePath::fromString("/temp/");
+        m_targetPath = "/temp/";
 
     m_timer.setInterval(500);
     m_timer.setSingleShot(false);