[UI] Redesigned "Install Content" window

- Added ID for each ImGui window
- Added filesystem::CreateFolder function

src/xenia/ui/imgui_dialog.h

@@ -40,6 +40,7 @@ class ImGuiDialog {
   ImGuiDrawer* imgui_drawer() const { return imgui_drawer_; }
   ImGuiIO& GetIO();
 
+  uint64_t GetWindowId() const { return next_window_id_; }
   // Closes the dialog and returns to any waiters.
   void Close();
 
@@ -48,6 +49,8 @@ class ImGuiDialog {
   virtual void OnDraw(ImGuiIO& io) {}
 
  private:
+  static uint64_t next_window_id_;
+
   ImGuiDrawer* imgui_drawer_ = nullptr;
   bool has_close_pending_ = false;
   std::vector<xe::threading::Fence*> waiting_fences_;