Allow to config/show cell borders

[shawn11ZX]

Cell's are a good way to organise different sections in a note.

But currently there is not visible boundary between cell, which makes it impossible to use cells that way.

[shawn11ZX]

Can be done by:

1. go to Preference -> Styles -> Editor -> Edit CSS
2. Add

   
   .text-cell {
   border: 1px solid black;
   }

.code-cell { border: 1px solid black; }

.latex-cell { border: 1px solid black; }

.markdown-cell { border: 1px solid black; }