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/boot2qt/qdbdevicedebugsupport.cpp

@@ -88,7 +88,7 @@ public:
         Runnable r = runnable();
 
         CommandLine cmd;
-        cmd.setExecutable(device()->mapToGlobalPath(FilePath::fromString(Constants::AppcontrollerFilepath)));
+        cmd.setExecutable(device()->filePath(Constants::AppcontrollerFilepath));
 
         if (m_useGdbServer) {
             cmd.addArg("--debug-gdb");