Bug 853933 - Don't inline all locales as JSON in the HTML

Currently, in order to ensure that apps are translated before first paint, all l10n resources are included as one multi-locales JSON dictionary, which is serialized in an inline `<script>` element for each HTML document.  This works fine when shipping few locales, but it can increase the document size quite badly for 60+ locales, thus delaying the startup time very significantly.

This patch allows to support 60+ locales more easily by having:
* one minimal JSON dictionary per locale in each HTML file;
* one extensive JSON dictionary per locale in each webapp.

The inline JSON dictionary is split on a per-locale basis (= one `<script>` node per supported locale) and only contains the l10n strings that are required to translate the HTML document: the "startup cost" of each locale becomes neglectable, and it removes the risk of high RAM consumption when parsing a giant multi-locale JSON at startup.

All l10n resources that are not declared in the HTML document are concatenated in a single JSON dictionary per locale: this reduces the number of disk I/O to load l10n resources and does not increase the total disk size of each webapp.  To enable this, this patch implements a `{{locale}}` quasi-litteral in the HTML markup that is used to declare l10n resources:
  <link rel="prefetch" type="application/l10n" href="/{{locale}}.json" />
where `{{locale}}` is replaced by the value of `navigator.language`.

Note that this latter optimization is only interesting for apps that wait for the `localized` event to start. With the inline JSON dictionary for the declarative content and the `mozL10n.localize()` method, most apps could probably stop waiting for this `localized` event now.
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Bug 881148 - hdpi/xhdpi resolution assets and layout for Gallery app - update images
Bug 889211 - [SMS] The delivery repory icon for the message is shown with two checks
Bug 889347 - SelectionChange event doesn't bubble through in mozKeyboard
Merge pull request #10762 from comoyo/keyboard_selectionend_gaia

Bug 889347 - SelectionChange event doesn't bubble through in mozKeyboard. r=xyuan
Bug 889175 - Fix UI problem of fullscreen app when call incoming
Bug 890079 - Homescreen grid tests failing in TBPL with: document.createTouch is not a function r=crdlc

don't try to send touch events if document.createTouch is missing.
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* Refactored how threads are added when a new message is received
* This incidentally resolves the bug
* made FixedHeader's refresh and scroll handler so that it's being called once
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* remove some unused code in FixedHeader
* adds tests for FixedHeader
* Fix indentation of createThreadMockup
Merge pull request #10786 from mozshiao9/master_sms_889211

Bug 889211 - [SMS] The delivery repory icon for the message is shown with two check, r=@steveck-chung
Merge pull request #10779 from alivedise/bugzilla/889224/remove-remember-me-for-fullscreen

Bug 889224 - Remove remember me option for fullscreen request, r=timdream, a=leo+
Merge pull request #10794 from alivedise/bugzilla/889175/incoming-call-statusbar

Bug 889175 - Fix UI problem of fullscreen app when call incoming, r=timdream, a=leo+
Merge pull request #10775 from cctuan/master-hd

Bug 881148 - hdpi/xhdpi resolution assets and layout for Gallery app
Merge pull request #10709 from fabi1cazenave/l10nOptim-bug853933

Bug 853933 - Don't inline all locales as JSON in the HTML, r=ochameau