Loading bar for local geoJson file upload and geoJsonHash request

[Grandro]

This implements a loading bar in the same fashion as the already existing general loading bar to display progress when uploading a geoJson file to the browser and sending the contents to the server.

We have the following steps:

1. The user selects a file to upload.
2. The file content is read using the FileReader class, loading bar adjusted with the onprogress event.
3. The read content is sent to the server to retrieve a geoJsonHash using XMLHttpRequest, loading bar adjusted with the onprogress event.
4. The frontend asks for the result using the obtained hash, the general loading bar shows up.

Note that there is also a method implemented in comments using TransformStream and fetch which I couldn't get to work. Also note that the width of the loading bar and the displayed percentage sometimes don't match during step 2). If I set a breakpoint in the debugger of my browser after the percentage and width has been altered, it does match. If someone has an idea why this could be happening I would gladly appreciate a hint.

Step 2) without breakpoints:

Step 2) with breakpoints: