Add proper support for inserting/removing characters in hex editor

[sirdarckcat]

At the moment deleting the char ascii value will delete the character.

It would be nice if we added support for inserting and deleting characters (and undoing). Would require a survey of other hex editors to see if there's a common pattern users are familiar with.

[sirdarckcat]

Some tools use a key combination to switch between insert mode and overwrite mode. Others allow the user to click on the gap between chars to insert a new one.

The key combination is a bit hard to discover, but a tab stop on the caret between chars would probably work well.

[sirdarckcat]

So the tab stop probably would have to be:

[hex] [insert hex] ... [char] [insert char]

If the insert hex/char is a button, then the user can only insert one character at a time, which might be frustrating. On the other hand, if it's an input, it might be confusing why it behaves differently from the other inputs.

One possible solution is to only allow insertion and deletion on the char view (as the third tab stop).