search: allow searching for some escape sequences without regex

Being able to search for \t and \n without regex is handy, so we'll copy
what gedit does here and enable it by default.

src/libide/editor/ide-editor-search.c

@@ -978,8 +978,13 @@ void
 ide_editor_search_set_search_text (IdeEditorSearch *self,
                                    const gchar     *search_text)
 {
+  g_autofree gchar *unescaped = NULL;
+
   g_return_if_fail (IDE_IS_EDITOR_SEARCH (self));
 
+  if (!ide_editor_search_get_regex_enabled (self))
+    search_text = unescaped = gtk_source_utils_unescape_search_text (search_text);
+
   gtk_source_search_settings_set_search_text (self->settings, search_text);
 
   /*