ProjectExplorer: Introduce a IDevice::filePath

Same function as mapToGlobalPath, but operating on a QString specifying
the local path part. It's the majority use case and arguably the 'right
thing' to use instead of a full FilePath that can refer to arbitrary
devices.

Change-Id: Ifc8bd340e2e8859fe549f5724eb94269f587c418
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/perfprofiler/perfprofilerruncontrol.cpp

@@ -147,7 +147,7 @@ public:
 
         Runnable perfRunnable = runnable();
 
-        CommandLine cmd({device()->mapToGlobalPath("perf"), {"record"}});
+        CommandLine cmd({device()->filePath("perf"), {"record"}});
         cmd.addArgs(m_perfRecordArguments);
         cmd.addArgs({"-o", "-", "--"});
         cmd.addCommandLineAsArgs(perfRunnable.command, CommandLine::Raw);