[UI] Redesigned "Install Content" window

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

src/xenia/ui/imgui_dialog.cc

@@ -16,9 +16,12 @@
 namespace xe {
 namespace ui {
 
+uint64_t ImGuiDialog::next_window_id_ = 0;
+
 ImGuiDialog::ImGuiDialog(ImGuiDrawer* imgui_drawer)
     : imgui_drawer_(imgui_drawer) {
   imgui_drawer_->AddDialog(this);
+  next_window_id_++;
 }
 
 ImGuiDialog::~ImGuiDialog() {