Write proper selection handling code

[obuchtala]

As here:
https://github.com/substance/science-writer/blob/2d906772519e939bbaf665e1617cd818fcc84715/src/state_handlers.js#L20

It is not a sufficient treatment of selections.
There are now 4 categories:
sel.isNull()
sel.isPropertySelection()
sel.isContainerSelection()
sel.isTableSelection()

As in the above case, the code suggests that you want to have property selections only.
A good way is to do either

if (sel.isPropertySelection()) {
...
} else {
 ...
}

or using a guard:

if (!sel.isPropertySelection()) {
  return;
}