In Firefox 54 runtime.onMessageExternal was implemented, however external
messaging still in progress.
Make sure that compatibility code is used for Firefox 54+ instead of
native runtime.onMessageExternal.
Also runtime.id is differs from i18n.getMessage('@@extension_id') in
Firefox so make sure to use first one.

Fixes: https://github.com/nE0sIghT/chrome-gnome-shell-mirror/issues/52
Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=780393

Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=780282

For same reason as in abddd4bb we should use window.postMessage for
in-frame communication.

Bug: https://github.com/nE0sIghT/chrome-gnome-shell-mirror/issues/35

Yuri Konotopov authored Feb 22, 2017 and Yuri Konotopov committed Feb 22, 2017

runtime.sendMessage will not send message to sender frame, so we should
use window.postMessage method to inter-frame communication.

Fixes: https://github.com/nE0sIghT/chrome-gnome-shell-mirror/issues/36

See-Also: https://github.com/nE0sIghT/chrome-gnome-shell-mirror/issues/31

Fixes: https://github.com/nE0sIghT/chrome-gnome-shell-mirror/issues/28

Fixes: https://github.com/nE0sIghT/chrome-gnome-shell-mirror/issues/27

Bug: https://github.com/nE0sIghT/chrome-gnome-shell-mirror/issues/27
See-Also: https://git.gnome.org/browse/extensions-web/commit/?id=214f03ee90f0f391d3d4fdec23feedf45e3d0507

See-Also: https://bugzilla.gnome.org/show_bug.cgi?id=777650
Fixes: https://github.com/nE0sIghT/chrome-gnome-shell-mirror/issues/26

Most translators do not use html so let's treat new line characters in
translation_credits as line break.
pt_BR is only language that uses both \n and <br> so let's remove
duplicate line break for that language.