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/plugins/texteditor/highlightersettings.cpp

@@ -74,7 +74,7 @@ QString findFallbackDefinitionsLocation()
             Utils::SynchronousProcess process;
             process.setTimeoutS(5);
             Utils::SynchronousProcessResponse response
-                    = process.runBlocking({Utils::FilePath::fromString(program), {"--prefix"}});
+                    = process.runBlocking({program, {"--prefix"}});
             if (response.result == Utils::SynchronousProcessResponse::Finished) {
                 QString output = response.stdOut();
                 output.remove(QLatin1Char('\n'));