Ratio of file list vs. right sidebar is not ideal in some viewports

[kulmann]

Steps to reproduce

1. Have a medium sized screen (e.g. 720p resolution)
2. Open the files app and the right sidebar

Expected behaviour

The file list uses the largest part of screen real estate.

Actual behaviour

The right sidebar takes roughly half the width, leaving the user with a pretty narrow file list although the right sidebar doesn't need that much space

For now sufficient to finetune the width of the right sidebar to take much less width in all viewports. Maybe even fixed width for the right sidebar, if it still looks reasonable on huge screens.