Utils: Replace FilePath::onDevice() by new FilePath::withMappedPath()

Basically a.onDevice(b) == b.withNewMappedPath(a), matching the order
of b.withNewPath(a).

Whether the (curretly docker-specific) path mapping is useful /there/, and
whether some of the calls are needed at all is dubious. I added some
FIXME and changed a few cases directly.

Change-Id: I7514736ce922f632f1f737bc496f6783389a42b6
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/libs/utils/terminalhooks.cpp

@@ -27,7 +27,7 @@ FilePath defaultShellForDevice(const FilePath &deviceRoot)
     if (shell.isEmpty())
         return shell;
 
-    return shell.onDevice(deviceRoot);
+    return deviceRoot.withNewMappedPath(shell);
 }
 
 class ExternalTerminalProcessImpl final : public TerminalInterface