Left and right sidepanel widths

For both left and right sidepanels:

• Opening the sidepanel for the first time, its width is less than the set minimum width: if you resize the sidepanel the width 'jumps' to the minimum width (and thus can't be made the size it was anymore)
  I was unable to reproduce the original issue, but decided to change the implementation as such that it should not happen anymore.
  Solution: Initially, the sidepanel must open with a width equal to the set minimum width or the last width stored in local-storage if it is bigger than the minimum width.

• The minimum width of the left sidepanel is too wide: change it to 290px (was 320px)
• The minimum width of the right sidepanel is too wide: change it to 400px (was 440px)