Make the tab key insert a tab char in the editor rather than select elements outside of it

At the moment if you press tab outside of some select places, including lists (where it indents) and some plain text macros like the code macro, it will act like any normal form and focus on elements outside of the editor.

For consistency and the ability indent/outdent other content without having to use the editor toolbar buttons we could change the behaviour to stay in the editor.

The 'default' behaviour should become the inserting of a tab, however there are some existing special cases such as in tables where it moves between cells that should be maintained.