Introduce dialogs

Major changes:

demos/ve/index.php
* Renamed ve-demo-content to ve-demo-editor

ve.init.mw.ViewPageTarget, ve.init.sa.Target
* Added handlers for dialog events

ve.ui.*Dialog.js
* Added skeleton classes for dialogs

ve.init.Target.js
* Create abstract class methods for Target.

ve.init.sa.Target.js
* Create Standalone target view methods.

ve.init.mw.Target.js
* Added MW specific target view methods.
* Integration action buttons are now added to the edit
view in the toolbar.

ve.Surface.js
* Simplified constructor, now requiring a target which contains the container

* Other changes include some documentation and code cleanup.

Bug: 39597
Change-Id: Iff39266bdd3052f34bda254ca407030dbbc81f26
More work on dialog classes and their mw specific handling.

Changes include:

VisualEditor.i18n.php, VisualEditor.php
* i18n labels for dialogs

ve.init.mw.ViewPageTarget.js
* Initial go at onOpenDialog and onCloseDialog methods

ve.init.Target.js
* Change calls to dialog hide & show to close & open

ve.ui.MetaDialog.js, ve.ui.ContentDialog.js
* Pass surface when constructing
* Add static title message property

ve.ui.Surface.css
* Set high z-index for toolbar for shadow to overlap dialog.

ve.ui.Dialog.js
* Extends EventEmitter class.
* Changed hide/show method names to open/close.
* Create base ui elements.

ve.Surface.js
* Create instance of meta dialog.

Change-Id: I867ca0546606eeb5e2ab7f612bb5af700ab877ec
Merge "Introduce dialogs"
Merge "More work on dialog classes and their mw specific handling."