gui: fix backward search in find dialog * find-dialog.cc(handle_backward_search_changed): new slot which toggles the text of the start from serach check box * find-dialog.cc(constructor): connect checked state signal of backward search checkbox to new slot andle_backward_search_changed, new icon * find-dialog.cc(search_next): fix cursor position for start from search/end and for backward search, show message box if no more matches are found * find-dialog.h: new slot handle_backward_search_changed