Replace dialog: create the SearchContext before the actual search

The object method handler gedit_replace_dialog_response() creates the
SearchContext if it is not already created.

In commands-search.c, replace_dialog_response_cb() must be called after
the SearchContext creation, to execute the actual find or replace.

If replace_dialog_response_cb() is called before, the SearchContext may
not be already created, and the find or replace doesn't work.
Replace dialog: do not bind the search settings

The reason of the previous behavior is now explained in the code.

The regex errors appear when a button is pressed, and disappear when the
search entry's text is modified. Before, a regex compilation error was
reported directly, as soon as the pattern is typed. This is possible if
we create a hidden SearchContext (without highlighting) besides the
normal SearchContext. This hidden SearchContext would be created
directly, and also on each document tab switch.