quartz: update the keymap only if the input method changed

and not on each keystroke, which for some IMs apparently caused a full
update on each keystroke, not just a check for changes. Patch from
Takuro Ashie, bug #698183.
quartz: remove check for keymap changes from update_keymap()

The function is now only called when the keymap has actually
changed. bug #698183.
(cherry picked from commit e62709da2ee6619f26c8446b545418e5f1b9e151)
filechooser: Fix misleading documentation for gtk_file_chooser_set_preview_widget()

https://bugzilla.gnome.org/show_bug.cgi?id=701174
Fix gtk_text_iter_forward_to_tag_toggle() for end iter

The function must return TRUE only if there is a tag toggle _after_ the
iter, not _at_ the iter. So for the end iter, the function must always
return FALSE.

Add also unit tests for gtk_text_iter_forward_to_tag_toggle().

https://bugzilla.gnome.org/show_bug.cgi?id=691266