Defer ._load_extras() as late as possible

Before this change, if we open without a file, the body of
._load_extras() would execute just after the more important
controllers have been deferred, in other words the extra controllers
would get instantiated before those in _open_with_welcome().
Remove dead code
Remove unused variable
Convert replacements to unicode at initialisation
Correctly initialise in clear_words() and reuse

(the initialisation for the variables were swapped around)
Remove incorrect .decode()

We already get a StringElem from the TextBox' signal.
Avoid unnecessary call to len(StringElem)
Fix incorrect use of name mangling (double underscore)