FileManager: Only set ~/Desktop as model root path in desktop mode

Setting it as model root path in DirectoryView::setup_model() for
windowed mode as well would cause an issue with the following:

- "open ~/Desktop"
- "FileManager ~/Desktop"
- "Show in FileManager..." from Desktop context menu

When viewing the Desktop as the initial path it would be the same and
on_path_change wasn't called, leading to various widgets and window
properties not being updated.

Fixes SerenityOS#3772.

Applications/FileManager/DirectoryView.cpp

@@ -158,7 +158,8 @@ const GUI::FileSystemModel::Node& DirectoryView::node(const GUI::ModelIndex& ind
 
 void DirectoryView::setup_model()
 {
- m_model->set_root_path(Core::StandardPaths::desktop_directory());
+ if (is_desktop())
+ m_model->set_root_path(Core::StandardPaths::desktop_directory());
 
  m_model->on_error = [this](int, const char* error_string) {
  auto failed_path = m_model->root_path();