Feature Request: filter server totals by selected folder

## Problem description

Currently if you select one folder everything gets filtered except for the server totals on the bottom of the left panel. I think it would be useful to have these stats correspond to the currently selected folder.

### Steps to reproduce

If you select all folders the stats are shown for all folders. Now select one of your folders. Stats on the bottom left are still calculated for the whole library.

## System information

* **Airsonic version**: 11.0.0-SNAPSHOT.20210226210819
* **Operating system**: Debian 10.9
* **Java version**: java 11.0.11
* **Proxy server**: Nginx 1.14.0
* **Client**: Firefox 88.0.1 x64
* **Language**: English