MainWindow: Decouple/rename Palette menu

[tetektoza]

Currently "Palette" menu isn't named too accurately, it contains actions for Palette, Translation and Unique Translation, this probably needs some sort of decoupling and moving it to separate menus OR renaming current "Palette" menu to something that by name would unify all those actions. Then, after this menu is renamed, a good idea would be to make separate menus nested in this menu, each one separately for Pallete, Translation and Unique Translation, and then all those actions that are available (for Palette, Translation and Unique Translation) right now could be moved into those menus.