Add min and max width to sidepanel

In addition to #262 

Irresponsible resizing is now prevented. The main body as well as the sidepanel need to have a minimal width, otherwise some visual issues can arise - and the user doesn't have any benefit of a ten pixel wide main body anyway.

The optimal min width setting is debatable though, but looks quite OK at 260px. Might be challenging for a mobile layout though, but that's another story anyway.