[User Experience] Persist sidebar width to preserve application restarts

[martinsefcik]

For me default sidebar width is small and after starting Bruno I have to always resize it which is annoying.
So, It would be fine to have it persisted in application settings on sidebar resize event and applied on application start.

[majkinetor]

While not explicitly asked for, this is a copy of #565. I have the very same issue with the sidebar.