Add menu items for dead keys

[1ec5]

Users new to typing in Vietnamese need some guidance about dead keys. The wiki page could be made more discoverable, but it isn’t interactive enough to really help people learn the syntax. Before taking over AVIM from Hiếu, I built an experimental IME extension that displayed all the supported dead keys in a rich tooltip. Google Input Tools sports a fancy on-screen keyboard, but that isn’t quite suitable for Vietnamese input methods, which interact with the entire word, not just the previous character.

AVIM’s menu should have an item for each dead key, with the keyboard shortcut alongside it. XUL <menuitem>s support displaying accelerator-less <key>s, even if those <key>s are nonfunctional. If I’m reading this code correctly, Firefox on the Mac also bridges accelerator-less shortcuts to native menu items.