Tuneit next

See merge request !4

feat: graph widgets, system monitor, async improvements

- Add info_graph, info_graph_large, info_graph_multi widgets
- Add system_monitor module with CPU/RAM/Swap monitoring
- Rework async value loading with ThreadPoolExecutor
- Pause update timer when page is hidden (map/unmap)
- Rework Entry/FileChooser reset button behavior
- Fix NumStepper int conversion when digits=0
- Update app icon and add artists to About (GNOME Design)

Requested-by: @fiersik