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/autotoolsprojectmanager/autogenstep.cpp

@@ -83,7 +83,7 @@ AutogenStep::AutogenStep(BuildStepList *bsl, Id id) : AbstractProcessStep(bsl, i
     });
 
     setCommandLineProvider([arguments] {
-        return CommandLine(FilePath::fromString("./autogen.sh"),
+        return CommandLine(FilePath("./autogen.sh"),
                            arguments->value(),
                            CommandLine::Raw);
     });