editor: implement cut/copy/paste and alternate translation attach

Cut, Copy, Paste and Attach Alternate Translation File were listed in the RC files but never wired up in code, so they never appeared in the menus.

I implemented cut/copy/paste using triggerFocusedEditAction() which routes the action to whatever widget has focus, falling back to the translation area if needed.

For attach alternate translation file, I added attachAlternateTranslationFileDialog() which opens a file picker and calls the existing attachAlternateTranslationFile().

Files touched:

src/editortab.cpp: triggerFocusedEditAction, cut/copy/paste wrappers, attachAlternateTranslationFileDialog slot

src/editortab.h: declare new private methods