Add menu mnemonic - File / &Export, etc
Ubuntu 14.04, GIT commit: c2c03e4
We should add mnemonics within each menu to make them easier to access. For example, File / Export should have the "E" of "Export" underlined, making it possible to invoke the command by "Alt+F E". This should require no real coding changes on our part; just inserting "&" in the names of menu items.