Add interplatform, i18n shortcuts to ui buttons.

Objective:
* Put command shortcuts in button title attributes. (Bug 42919)
* Provide a registry for platform specific command triggers and their
corresponding i18n messages. (Bug 44012)
* Enable loading of triggers after ve.Surface is created. (lazy load)

Changes:

VisualEditor.i18n.php
* Add default trigger i18n messags for mac and pc system platforms

VisusalEditor.php, demos/ve/index.php
* Add links to files

ve.init.mw.Platform.js
* Define getUserLanguage and getSystemPlatform methods.

ve.init.sa.Platform.js
* Define getUserLanguage and getSystemPlatform methods.

ve.init.Platform.js
* Define abstract methods: getUserLangauge, getSystemPlatform

ve.ui.BoldBUttonTool.js, ve.ui.IndentButtonTool.js, ve.ui.ItalicButtonTool.js,
ve.ui.LinkButtonTool.js, ve.ui.OutdentButtonTool.js, ve.ui.RedoButtonTool.js,
ve.ui.UndoButtonTool.js
* Add registration for command triggers.

ve.Surface.js
* Methodize loading of triggers.
* Bind register event to ve.triggerRegistry to allow lazy loading of triggers.

ve.ui.Tool.js
* Init pre-registered tooltip messages.
* Update tool titles when new triggers are loaded.

ve.CommandRegistry.js
* Remove command registration ( moved to buttons themselves )

ve.TriggerRegistry.js
* New class for registering triggers.

ve.init.mw.ViewPageTarget.js
* Changed instance of unindent command to outdent.

Change-Id: Id8580a3f81aac751db0b7482422a73912648dfed
Merge "Add interplatform, i18n shortcuts to ui buttons."