Left curly brace shouldn't trigger Macro Autocomplete if an existing macro is selected

To reproduce:

1. Insert a macro in the RTE, eg Section macro
2. Select the text of the macro start tag
3. Press { - the macro autocomplete starts with the start tag selected

We need to decide on a behaviour for this that won't surprise the user - for example, just type the character at the cursor position. It will need extending to the link and image autocompletes also.