mapkey is ignored in mappings with key-modifiers and in link-hint mode

Hello!

You recently added some hotkey localization mechanism and that is cool! Thank you. Yet the implementation has some flows:

• after pressing the f button the user has to switch keyboard layout in order to type link hints in English, i.e. mapkey does not work in this case.
• try to define command like map <a-k> nextTab and this will work in English layout only, so the user has to define a duplicate mapping, e.g. map <a-л> nextTab for Russian layout.

Chromium 56.0.2924.87 (Developer Build) (64-bit)
Vimium: 1.58.1

By the way, I've implemented another langmapping mechanism a couple of days ago (it works both for <a-…> mappings and with link following mode) and were wanting to pull-request just before noticing you've done the mapkey :) Not sure if I should pull-request now, but please look at my implementation: https://github.com/lxyd/vimium-mod/commit/8870da55dc1a58f735ed2432f55e1d3f376d3b54

Reproduce the mapkey behavior with key-modifier mappings such as <a-k> and in link-hint mode, including a non-English keyboard layout. Trace the existing mapkey, key-modifier, and link-hint handling, then verify that both mapping types work without duplicate layout-specific mappings.