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/libs/utils/qtcprocess.cpp

@@ -535,7 +535,7 @@ void QtcProcess::setUseCtrlCStub(bool enabled)
 
 void QtcProcess::start(const QString &cmd, const QStringList &args)
 {
-    setCommand({cmd, args});
+    setCommand({FilePath::fromString(cmd), args});
     start();
 }