gui: cleanup some string constants for translations * file-editor.h: deleted constants for file filters and name for new files * file-editor.cc (request_open_file): translate file filter in open dialog * file-editor.cc (request_new_file): create new edit tab with empty title because the title is updated later * file-editor-tab.cc (update_window_title): translate unnamed title * file-editor-tab.cc (save_file_as): translate file filter, shorter warning message when new file name is the same as the current one

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