Add mnemonics (hotkeys) to menu bar. Resolve #1068.

[KonradHoeffner]

Use the first character of each menu item as a mnemonic. This allows for example to press Alt+F to enter the file menu. This does not add a workable mnemonic to the Reasoner menu, because the R is already taken by Refactor. According to https://bugs.openjdk.org/browse/JDK-4515762 that should actually work since 2012, but even if one of them doesn't work it is still a large step up from having no mnemonic at all. A more generic approach would be to extend the plugin XML files with an extra "mnemonic" attribute and support that, but that would be a much larger change. This also adds a few mnemonics to inner parent menus but I think that is a positive change anyways.