Utils: Add CommandLine convenience constructors

... taking a QString for the executable.

This weakens the very explicit QString -> FileName conversion via the
named constructors for the special case of constructing a CommandLine.

I think that's worthwhile here, as it reduces the noise on the caller
site under circumstance where the nature of the thing is obvious.

Change-Id: I27b4a73639728893d053b2e7ba65cb745f0ffe83
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/libs/utils/fileutils.h

@@ -135,7 +135,9 @@ public:
     enum class MetaCharMode { Abort, Ignore };
 
     CommandLine() {}
+    explicit CommandLine(const QString &executable);
     explicit CommandLine(const FilePath &executable);
+    CommandLine(const QString &exe, const QStringList &args);
     CommandLine(const FilePath &exe,
                 const QStringList &args,
                 MetaCharMode metaCharMode = MetaCharMode::Abort);