Fix new paragraph behavior in Firefox

[jefago]

Fixes #89.

When inserting a new paragraph, Firefox splits the div element in two, whereas Chromium inserts \n\n.

Changes:

• When comparing which lines have changed (ie are dirty) after a paragraph insertion / deletion, also take into account the class name
• Fix continuable type treatment in Firefox (don't return early when selection is at the beginning of the row after a paragraph break)