Improve toolbar and editor interactions

This brings 2 main things:

• The toolbar actions are now checked based on what the cursor is hovering/the selected text. Unchecking those actions will remove the related delims. And checking them will add the delims. Note: Non-block delims (bold, italic, ...) won't be added if no text is selected.

• tab and return handling in the editor have been move to fully use the acces to the QTextDocument. As a bonus, pressing shift+return will result in a <br>\n and pressing alt+return will add a horizontal rule after the block of text.