Utils: Replace FilePath::rootPath() by some isRootPath()

Keeps more context to make it host-independent later.

Change-Id: I0f8ad3e8794daa6324662847203200ae378d34b4
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/libs/utils/filepath.cpp

@@ -167,9 +167,10 @@ FilePath FilePath::currentWorkingPath()
     return FilePath::fromString(QDir::currentPath());
 }
 
-FilePath FilePath::rootPath()
+bool FilePath::isRootPath() const
 {
-    return FilePath::fromString(QDir::rootPath());
+    // FIXME: Make host-independent
+    return operator==(FilePath::fromString(QDir::rootPath()));
 }
 
 QString FilePath::encodedHost() const