Pretty Loading screen

[sfeilmeier]

From openems-gui created by sfeilmeier : OpenEMS/openems-gui#22

Bug Report or Feature Request (mark with an x)

- [ ] bug report -> please search issues before submitting
- [x] feature request

Repro steps.

Start the application and wait till it loads. You will see "Loading...".

Desired functionality.

There should be some nice Loading visualization.